ci: build ep_etherpad-lite before resolving plugins

The 'with plugins' jobs install ep_markdown / ep_readonly_guest / etc.
which require ep_etherpad-lite at install-time. The dist + dist-cjs
twins must exist before pnpm resolves those subpath imports.

Also run check:exports as a fast canary before plugin install.

Author: SamTV12345

.github/workflows/backend-tests.yml

@@ -131,6 +131,14 @@ jobs:
       - name: Build admin ui
         working-directory: admin
         run: pnpm build
+      -
+        name: Build ep_etherpad-lite (dist + dist-cjs)
+        working-directory: src
+        run: pnpm run build
+      -
+        name: Verify exports map
+        working-directory: src
+        run: pnpm run check:exports
       -
         name: Install Etherpad plugins
         run: >
@@ -264,6 +272,14 @@ jobs:
       - name: Build admin ui
         working-directory: admin
         run: pnpm build
+      -
+        name: Build ep_etherpad-lite (dist + dist-cjs)
+        working-directory: src
+        run: pnpm run build
+      -
+        name: Verify exports map
+        working-directory: src
+        run: pnpm run check:exports
       -
         name: Install Etherpad plugins
         run: >