add build cache action

Signed-off-by: ahuston-0 <aliceghuston@gmail.com>

.github/workflows/flake-update.yml

@@ -5,68 +5,25 @@ on:
   schedule:
     - cron: "00 12 * * *"
 jobs:
-  createPullRequest:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v4
-      # - name: Login to Docker Hub
-        # uses: docker/login-action@v3
-        # with:
-        #   username: ${{ secrets.DOCKERHUB_USERNAME }}
-        #   password: ${{ secrets.DOCKERHUB_TOKEN }}
-      - name: Install Nix
-        uses: cachix/install-nix-action@v24
-        with:
-          extra_nix_config: |
-            experimental-features = nix-command flakes auto-allocate-uids configurable-impure-env
-      - name: Calculate pre-drv
-        run: nix run git+https://nayeonie.com/ahuston-0/flake-update-diff -- --build .
-      # - name: Pull latest docker images
-      #   run: nix ./utils/fetch-docker.sh
-      - name: Update flake.lock (part 1)
-        run: nix flake update
-      - name: Calculate post-drv
-        run: nix run git+https://nayeonie.com/ahuston-0/flake-update-diff --  --build .
-      # - name: Calculate diff
-      #   run: nix ./utils/diff-evals.sh
-      # - name: Read diff into environment
-      #   run: |
-      #     delimiter="$(openssl rand -hex 8)"
-      #     {
-      #     echo "POSTDIFF<<${delimiter}"
-      #     cat post-diff
-      #     echo "${delimiter}"
-      #     } >> $GITHUB_ENV
-
-
-      # - name: Restore flake.lock for next step
-      #   run: git restore flake.lock
-      # - name: Update flake.lock
-      #   id: update
-      #   uses: DeterminateSystems/update-flake-lock@main
-      #   with:
-      #     token: ${{ secrets.GH_TOKEN_FOR_UPDATES }}
-      #     pr-body: |
-      #       Automated changes by the [update-flake-lock](https://github.com/DeterminateSystems/update-flake-lock) GitHub Action.
-
-      #       ```
-      #       {{ env.GIT_COMMIT_MESSAGE }}
-      #       ```
-
-      #       ```
-      #       {{ env.POSTDIFF }}
-      #       ```
-      #     pr-labels: |                  # Labels to be set on the PR
-      #       dependencies
-      #       automated
   update_lockfile:
     runs-on: ubuntu-latest
     if: github.ref == 'refs/heads/main' # ensure workflow_dispatch only runs on main
     steps:
       - name: Checkout repository
         uses: actions/checkout@v4
+
       - name: Install nix
         uses: https://github.com/DeterminateSystems/nix-installer-action@main
+
+      - name: Restore Nix store
+        id: restore
+        uses: nix-community/cache-nix-action@v6
+        with:
+          # save a new cache every time `ci.yaml` changes
+          primary-key: similar-cache-${{ matrix.os }}-individual-${{ hashFiles('flake.lock', '*.nix') }}
+          # otherwise, restore a common cache if and only if it matches the current `ci.yaml`
+          restore-prefixes-first-match: similar-cache-${{ matrix.os }}-common-
+
       - name: Setup Attic cache
         uses: ryanccn/attic-action@v0
         with:
@@ -74,8 +31,10 @@ jobs:
           cache: ${{ secrets.ATTIC_CACHE }}
           token: ${{ secrets.ATTIC_TOKEN }}
           skip-push: "true"
+
       - name: Get pre-snapshot of evaluations
         run: nix ./utils/eval-to-drv.sh pre
+
       - name: Update flake.lock
         id: update
         run: |
@@ -90,10 +49,13 @@ jobs:
           echo "EOF" >> $GITHUB_ENV
 
           rm update.log
+
       - name: Get post-snapshot of evaluations
         run: nix ./utils/eval-to-drv.sh post
+
       - name: Calculate diff
         run: nix ./utils/diff-evals.sh
+
       - name: Read diff into environment
         run: |
           delimiter="$(openssl rand -hex 8)"
@@ -102,6 +64,7 @@ jobs:
           cat post-diff
           echo "${delimiter}"
           } >> $GITHUB_ENV
+
       - name: Write PR body template
         uses: https://github.com/DamianReeves/write-file-action@v1.3
         with:
@@ -128,6 +91,7 @@ jobs:
         with:
           files: "pr_body.template"
           output-filename: "pr_body.md"
+
       - name: Save PR body
         id: pr_body
         uses: juliangruber/read-file-action@v1
@@ -161,9 +125,25 @@ jobs:
           pr-labels: |                  # Labels to be set on the PR
             dependencies
             automated
+
       - name: Push to Attic
         run: nix ./utils/attic-push.bash
         continue-on-error: true
+
+      - name: Save Nix store
+        uses: nix-community/cache-nix-action@v6
+        with:
+          # save a new cache every time `ci.yaml` changes
+          primary-key: similar-cache-${{ matrix.os }}-individual-${{ hashFiles('flake.lock', '*.nix') }}
+          # do purge caches
+          purge: true
+          # purge all versions of the individual cache
+          purge-prefixes: similar-cache-${{ matrix.os }}-individual-
+          # created more than 0 seconds ago relative to the start of the `Post Restore` phase
+          purge-created: 0
+          # except the version with the `primary-key`, if it exists
+          purge-primary-key: never
+
       - name: Print PR number
         run: |
           echo "Pull request number is ${{ steps.create-pull-request.outputs.pull-request-number }}."