README: demonstrate a weekly flake.lock update

It's arguably not that useful to be inundated with flake.lock update PRs
every day, so do it once a week on Sunday.

README.md

@@ -2,7 +2,7 @@
 
 This is a GitHub Action that will update your flake.lock file whenever it is run.
 
-> NOTE: We hardcode the `install_url` to a relatively recent `nixUnstable` (`nix-2.5pre20211015_130284b` currently). If you need a newer version that includes a new feature or important bug fix, feel free to file an issue or send a PR bumping the `install_url` inside the [`action.yml`](action.yml)!
+> **NOTE:** As of v3, this action will no longer automatically install Nix to the action runner. You **MUST** set up a Nix with flakes support enabled prior to running this action, or your workflow will not function as expected.
 
 ## Example
 
@@ -11,7 +11,9 @@ An example GitHub Action workflow using this action would look like the followin
 ```yaml
 name: update-flake-lock
 on:
-  workflow_dispatch:
+  workflow_dispatch: # allows manual triggering
+  schedule:
+    - cron: '0 0 * * 0' # runs weekly on Sunday at 00:00
 
 jobs:
   lockfile:
@@ -19,8 +21,27 @@ jobs:
     steps:
       - name: Checkout repository
         uses: actions/checkout@v2
+      - name: Install Nix
+        uses: cachix/install-nix-action@v14
+        with:
+          install_url: https://nixos-nix-install-tests.cachix.org/serve/vij683ly7sl95nnhb67bdjjfabclr85m/install
+          install_options: '--tarball-url-prefix https://nixos-nix-install-tests.cachix.org/serve'
+          extra_nix_config: |
+            experimental-features = nix-command flakes
+            access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
       - name: Update flake.lock
-        uses: DeterminateSystems/update-flake-lock@main
+        uses: DeterminateSystems/update-flake-lock@v3
+```
+
+## Running GitHub Actions CI
+
+GitHub Actions will not run workflows when a branch is pushed by or a PR is opened by a GitHub Action. To work around this, try:
+
+```
+git branch -D update_flake_lock_action
+git fetch origin
+git checkout update_flake_lock_action
+git commit --amend --no-edit
+git push origin update_flake_lock_action --force
 ```
 
-To have this workflow run on a recurring basis, see the [GitHub Actions documentation on the `schedule` key](https://docs.github.com/en/actions/learn-github-actions/events-that-trigger-workflows#schedule).

action.yml

@@ -3,11 +3,6 @@ description: 'Update your flake.lock and send a PR'
 runs:
   using: "composite"
   steps:
-    - uses: cachix/install-nix-action@v14
-      with:
-        install_url: https://github.com/numtide/nix-unstable-installer/releases/download/nix-2.5pre20211015_130284b/install
-        extra_nix_config: |
-          experimental-features = nix-command flakes
     - run: nix flake update --commit-lock-file
       shell: bash
       env:
@@ -15,10 +10,37 @@ runs:
         GIT_AUTHOR_EMAIL: <github-actions[bot]@users.noreply.github.com>
         GIT_COMMITTER_NAME: github-actions[bot]
         GIT_COMMITTER_EMAIL: <github-actions[bot]@users.noreply.github.com>
+    - run: |
+        content="$(git log --format=%b -n 1)"
+        content="${content//'%'/'%25'}"
+        content="${content//$'\n'/'%0A'}"
+        content="${content//$'\r'/'%0D'}"
+        echo "::set-output name=msg::$content"
+      shell: bash
+      id: commit_message
     - name: Create PR
       uses: peter-evans/create-pull-request@v3
       with:
         branch: update_flake_lock_action
         delete-branch: true
         title: "flake.lock: Update"
-        body: Automated changes by the [update-flake-lock](https://github.com/DeterminateSystems/update-flake-lock) GitHub Action.
+        body: |
+          Automated changes by the [update-flake-lock](https://github.com/DeterminateSystems/update-flake-lock) GitHub Action.
+
+          ```
+          ${{ steps.commit_message.outputs.msg }}
+          ```
+
+          ### Running GitHub Actions on this PR
+
+          GitHub Actions will not run workflows on pull requests which are opened by a GitHub Action.
+
+          To run GitHub Actions workflows on this PR, run:
+
+          ```sh
+          git branch -D update_flake_lock_action
+          git fetch origin
+          git checkout update_flake_lock_action
+          git commit --amend --no-edit
+          git push origin update_flake_lock_action --force
+          ```