Improve CI with builtin checkout and gem caching

Use the runner's builtin:checkout so jobs need no action download or
Node for checkout, cancel superseded runs with concurrency, cache the
plugin's gems in the lint job, and prefer the pnpm store.

.gitea/workflows/ci.yml

@@ -6,6 +6,10 @@ on:
       - main
   pull_request:
 
+concurrency:
+  group: ci-${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
 env:
   RAILS_ENV: test
   PGUSER: discourse
@@ -22,13 +26,23 @@ jobs:
       - name: Set working directory owner
         run: chown -R root:root .
 
-      - uses: actions/checkout@v4
+      - uses: builtin:checkout
+
+      - name: Cache gems
+        uses: actions/cache@v4
+        with:
+          path: vendor/bundle
+          key: ${{ runner.os }}-lint-gems-${{ hashFiles('Gemfile.lock') }}
+          restore-keys: |
+            ${{ runner.os }}-lint-gems-
 
       - name: Install JS dependencies
-        run: pnpm install --frozen-lockfile
+        run: pnpm install --frozen-lockfile --prefer-offline
 
       - name: Bundle install
-        run: bundle install
+        run: |
+          bundle config set --local path vendor/bundle
+          bundle install
 
       - name: Rubocop
         run: bundle exec rubocop .
@@ -49,7 +63,7 @@ jobs:
         run: chown -R root:root .
 
       - name: Checkout plugin
-        uses: actions/checkout@v4
+        uses: builtin:checkout
         with:
           path: plugin
 
@@ -119,7 +133,7 @@ jobs:
 
       - name: Install JS dependencies
         working-directory: core
-        run: pnpm install --frozen-lockfile
+        run: pnpm install --frozen-lockfile --prefer-offline
 
       - name: Restore database from cache
         if: steps.app-cache.outputs.cache-hit == 'true'