Fix workflow concurrency to prevent PR builds from cancelling main builds

Previously, PR builds and main branch push builds shared the same
concurrency group because pull_request_target events use the base
branch ref. This caused PR builds to cancel ongoing main builds.

Now PR builds use a unique concurrency group (pr-<number>) while
push builds continue to use the branch ref, preventing interference.

Co-Authored-By: Claude <noreply@anthropic.com>