Install action's own gems via ruby/setup-ruby before running

The exe needs octokit, which isn't in the consumer's Gemfile. A second
ruby/setup-ruby step pointed at the action's own Gemfile handles the
install and caches it keyed on Gemfile.lock. BUNDLE_GEMFILE in the run
step ensures bundle exec picks up the action's gems rather than the
consumer's.

Author: neilvcarvalho

action.yml

@@ -40,21 +40,28 @@ inputs:
 runs:
   using: "composite"
   steps:
-    - name: Set up Ruby
+    - name: Set up Ruby (consumer)
       uses: ruby/setup-ruby@v1
       with:
         bundler-cache: true
         ruby-version: ${{ inputs.ruby-version }}
 
+    - name: Set up Ruby (action)
+      uses: ruby/setup-ruby@v1
+      with:
+        bundler-cache: true
+        working-directory: ${{ github.action_path }}
+
     - name: Run importmap-update
       shell: bash
       working-directory: ${{ inputs.working-directory }}
       env:
+        BUNDLE_GEMFILE:           ${{ github.action_path }}/Gemfile
         GITHUB_TOKEN:             ${{ inputs.github-token }}
         GITHUB_REPOSITORY:        ${{ github.repository }}
         INPUT_CONFIG_FILE:        ${{ inputs.config-file }}
         IMPORTMAP_BASE_BRANCH:    ${{ inputs.base-branch }}
         IMPORTMAP_DRY_RUN:        ${{ inputs.dry-run }}
         IMPORTMAP_AUTHOR_NAME:    ${{ inputs.author-name }}
         IMPORTMAP_AUTHOR_EMAIL:   ${{ inputs.author-email }}
-      run: ${{ github.action_path }}/exe/importmap-update 2>&1 | tee "${IMPORTMAP_RUN_LOG:-/dev/null}"
+      run: bundle exec ${{ github.action_path }}/exe/importmap-update 2>&1 | tee "${IMPORTMAP_RUN_LOG:-/dev/null}"

exe/importmap-update

@@ -162,7 +162,12 @@ when :run
     exit 2
   end
 
-  runner = Importmap::Update::Commands::ShellRunner.new
+  # Unset BUNDLE_GEMFILE for all child processes (bin/importmap, git, …).
+  # The action is invoked with BUNDLE_GEMFILE pointing at its own Gemfile so
+  # that `bundle exec` loads octokit; if that leaks into bin/importmap the
+  # Rails app's boot.rb will try to use the action's Gemfile instead of its
+  # own, which doesn't have Rails.
+  runner = Importmap::Update::Commands::ShellRunner.new(env: {"BUNDLE_GEMFILE" => nil})
   gh = Importmap::Update::GitHubClient.new(repo: repo, token: token)
   git = Importmap::Update::GitClient.new(
     runner: runner,