Fix builds failing to checkout: stop deleting tags, fix tag passing

Two issues causing all builds to fail:

1. Cleanup steps deleted git tags along with releases. Since builds
   are dispatched asynchronously, they tried to checkout tags that
   had already been deleted. Now cleanup only deletes releases (which
   frees storage by removing assets) but preserves git tags.

2. Linux/macOS build workflows used $GITHUB_OUTPUT step outputs for
   the tag, which is unreliable on Gitea runners. Switched to the
   same job-level env var pattern (RELEASE_TAG) that works on Windows.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitea/workflows/release.yml

@@ -159,9 +159,7 @@ jobs:
             echo "  Deleting release ${TAG} (ID: ${ID})..."
             curl -s -X DELETE -H "Authorization: token ${BUILD_TOKEN}" \
               "${REPO_API}/releases/${ID}"
-
-            # Also delete the tag
-            curl -s -X DELETE -H "Authorization: token ${BUILD_TOKEN}" \
-              "${REPO_API}/tags/${TAG}"
+            # Keep the git tag -- only delete the release (assets).
+            # Deleting tags breaks builds that haven't checked out yet.
           done
           echo "Cleanup complete"