Switch back to avoid messing up caches

.github/workflows/ci.yaml

@@ -47,14 +47,51 @@ jobs:
         run: npm run review
       - name: Test
         run: npm run test
-      - name: Generation Test
-        run: npm run test:gen && mv generated generated-$(git branch --show-current)
+
+  diff-generation:
+    permissions:
+      checks: write
+      pull-requests: write
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Use Node.js 20
+        uses: actions/setup-node@v4
+        with:
+          node-version: 20
+
+      # Re-use node_modules between runs until package-lock.json changes.
+      - name: Cache node_modules
+        id: cache-node_modules
+        uses: actions/cache@v4
+        with:
+          path: node_modules
+          key: node_modules-${{ hashFiles('package-lock.json') }}
+
+      # Re-use ~/.elm between runs until elm.json or review/elm.json changes.
+      # The Elm compiler saves downloaded Elm packages to ~/.elm saves
+      # downloaded tool executables there.
+      - name: Cache ~/.elm
+        uses: actions/cache@v4
+        with:
+          path: ~/.elm
+          key: elm-${{ hashFiles('elm.json', 'review/elm.json') }}
+
+      # Install npm packages, unless we restored them from cache.
+      # Since `npm ci` removes the node_modules folder before running it’s
+      # important to skip this step if cache was restored.
+      - name: npm ci
+        if: steps.cache-node_modules.outputs.cache-hit != 'true'
+        run: npm ci
+
+      - name: Generate from branch
+        run: npm run test:gen && mv generated generated-$GITHUB_HEAD_REF
 
       - name: Generate from main
-        run: git switch main && npm run test:gen && mv generated generated-main
+        run: git switch main && npm run test:gen && mv generated generated-main && git switch $GITHUB_HEAD_REF
 
       - name: Diff the outputs
-        run: echo '```diff' > diff.md && diff -w -d -r generated-main generated-$(git branch --show-current) >> diff.md && echo '```' >> diff.md
+        run: echo '```diff' > diff.md && diff -w -d -r generated-main generated-$GITHUB_HEAD_REF >> diff.md && echo '```' >> diff.md
 
       - name: Post a comment with the diff
         uses: JoseThen/[email protected]