ci(e2e): add BUI build step to E2E workflows

The @backstage/ui package has a custom build script that is not
recognized by `yarn backstage-cli repo build`, so it gets skipped.
This caused E2E tests to fail when packages import JavaScript
components from @backstage/ui.

Add explicit BUI build step to Linux and Windows E2E workflows,
matching the pattern already used in ci.yml and deploy_packages.yml.

Signed-off-by: Johan Persson <johanopersson@gmail.com>

.github/workflows/verify_e2e-linux.yml

@@ -66,6 +66,8 @@ jobs:
 
       - run: yarn tsc
       - run: yarn backstage-cli repo build
+        # BUI has a custom build script and needs to be built separately
+      - run: yarn --cwd packages/ui build
       - name: run E2E test
         run: |
           sudo sysctl fs.inotify.max_user_watches=524288

.github/workflows/verify_e2e-windows.yml

@@ -86,6 +86,8 @@ jobs:
 
       - run: yarn tsc
       - run: yarn backstage-cli repo build
+        # BUI has a custom build script and needs to be built separately
+      - run: yarn --cwd packages/ui build
       - name: run E2E test
         run: yarn e2e-test run
         env: