refactor(ci): Lint PR diffs only (#4137)

* ✨ Set up `eslint-plugin-diff`

* 🔧 Fix setup

* ⬆️ Upgrade `checkout` and `setup-node`

* 🔧 Fix YAML

* 🧪 Verify `eslint-plugin-diff` behavior

* Revert "🧪 Verify `eslint-plugin-diff` behavior"

This reverts commit 1fcc6c82996f89c2be88899978524b00b898ee8f.

* ✏️ Fix comment

.github/workflows/ci-pull-requests.yml

@@ -13,18 +13,27 @@ jobs:
         node-version: [14.x, 16.x]
 
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v3
+
       - name: Use Node.js ${{ matrix.node-version }}
         uses: actions/setup-node@v3
         with:
           node-version: ${{ matrix.node-version }}
           cache: 'npm'
-      - name: npm install, build, and test
-        run: |
-          npm install -g npm@latest
-          npm install
-          npm run build --if-present
-          npm test
-          npm run lint
+
+      - name: Install npm and dependencies
+        run: npm install -g npm@latest && npm install
+
+      - name: Build
+        run: npm run build --if-present
+
+      - name: Test
+        run: npm run test
+
+      - name: Fetch base branch for `git diff`
+        run: git fetch origin ${{ github.event.pull_request.base.ref }}:${{ github.event.pull_request.base.ref }}
+
+      - name: Run ESLint on changes only
         env:
-          CI: true
+          ESLINT_PLUGIN_DIFF_COMMIT: ${{ github.event.pull_request.base.ref }}
+        run: npm run lint