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
 
@@ -13,7 +13,7 @@ name: update-flake-lock
 on:
   workflow_dispatch: # allows manual triggering
   schedule:
-    - cron: '0 0 * * *' # runs daily at 00:00
+    - cron: '0 0 * * 0' # runs weekly on Sunday at 00:00
 
 jobs:
   lockfile:
@@ -21,7 +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@v1
+        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
 ```
 

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:
@@ -35,3 +30,17 @@ runs:
           ```
           ${{ 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
+          ```