add build cache action

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

.github/workflows/cache-merge.yml

@@ -0,0 +1,89 @@
+name: Nix CI
+on:
+  push:
+    # don't run on tags, run on commits
+    # https://github.com/orgs/community/discussions/25615
+    tags-ignore:
+      - "**"
+    branches:
+      - main
+  pull_request:
+    branches: ["main"]
+  merge_group:
+  schedule:
+    - cron: 0 0 * * *
+  workflow_dispatch:
+
+jobs:
+  # Merge similar `individual` caches
+  # Purge `individual` caches and old `common` caches
+  # Save new `common` caches
+  merge-similar-caches:
+    name: Merge similar caches
+    needs: make-similar-caches
+    strategy:
+      matrix:
+        os:
+          - [ubuntu-latest]
+    runs-on: ${{ matrix.os }}
+    steps:
+      - name: Checkout this repo
+        uses: actions/checkout@v4
+
+      - name: Install nix
+        uses: https://github.com/DeterminateSystems/nix-installer-action@main
+
+      - uses: nix-community/cache-nix-action@v6
+        name: create and purge common cache
+        with:
+          primary-key: similar-cache-${{ matrix.os }}-common-${{ hashFiles('flake.lock') }}
+          # if no hit on the primary key, restore individual caches that match `ci.yaml`
+          restore-prefixes-all-matches: |
+            similar-cache-${{ matrix.os }}-individual-${{ hashFiles('flake.lock', '*.nix') }}
+          # do purge caches
+          purge: true
+          # purge old versions of the `common` cache and any versions of individual caches
+          purge-prefixes: |
+            similar-cache-${{ matrix.os }}-common-
+          # 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
+
+      - uses: nix-community/cache-nix-action@v6
+        name: purge some individual caches
+        with:
+          primary-key: similar-cache-${{ matrix.os }}-common-${{ hashFiles('flake.lock') }}
+          # if no hit on the primary key, restore individual caches that match `ci.yaml`
+          restore-prefixes-all-matches: |
+            similar-cache-${{ matrix.os }}-individual-${{ hashFiles('flake.lock', '*.nix') }}
+          # do purge caches
+          purge: true
+          # purge old versions of the `common` cache and any versions of individual caches
+          purge-prefixes: |
+            similar-cache-${{ matrix.os }}-individual-
+          # created more than 0 seconds ago relative to the start of the `Post Restore` phase
+          purge-created: 259200
+          # except the version with the `primary-key`, if it exists
+          purge-primary-key: never
+
+  # Check that the `common` cache is restored correctly
+  merge-similar-caches-check:
+    name: Check a `common` cache is restored correctly
+    needs: merge-similar-caches
+    strategy:
+      matrix:
+        os:
+          - [ubuntu-latest]
+    runs-on: ${{ matrix.os }}
+    steps:
+      - name: Checkout this repo
+        uses: actions/checkout@v4
+
+      - name: Install nix
+        uses: https://github.com/DeterminateSystems/nix-installer-action@main
+
+      - name: Restore Nix store
+      - uses: nix-community/cache-nix-action@v6
+        with:
+          primary-key: similar-cache-${{ matrix.os }}-common-${{ hashFiles('flake.lock') }}