cache .next/cache between unit tests (#23415)

Part of #1246

.github/workflows/test.yml

@@ -70,15 +70,20 @@ jobs:
       - name: Install dependencies
         run: npm ci
 
-      - if: ${{ github.repository == 'github/docs-internal' }}
-        name: Clone early access
-        run: npm run heroku-postbuild
+      - name: Clone early access
+        if: ${{ github.repository == 'github/docs-internal' }}
+        run: script/early-access/clone-for-build.js
         env:
           DOCUBOT_REPO_PAT: ${{ secrets.DOCUBOT_REPO_PAT }}
           GIT_BRANCH: ${{ github.head_ref || github.ref }}
 
-      - if: ${{ github.repository != 'github/docs-internal' }}
-        name: Run build script
+      - name: Cache nextjs build
+        uses: actions/cache@c64c572235d810460d0d6876e9c705ad5002b353
+        with:
+          path: .next/cache
+          key: ${{ runner.os }}-nextjs-${{ hashFiles('package*.json') }}-${{ hashFiles('.github/workflows/test.yml') }}
+
+      - name: Run build script
         run: npm run build
 
       - name: Run tests