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>
Johan Persson
62318ac5c9