CI: use gitea.actor as registry login username

Gitea authenticates registry pushes by token alone; a single
REGISTRY_TOKEN secret suffices.

SPEC.md

@@ -186,9 +186,9 @@ are new.
   2. on a version tag only (`vX.Y.Z`, enforced): build the image with buildx
      and push it to the Gitea registry
      `git.rambossek.at/<owner>/gpu-turnstile` tagged `:<tag>` and `:latest`.
-     Login uses the repo secrets `REGISTRY_USERNAME` / `REGISTRY_TOKEN` (an
-     access token with `write:package` scope) because the automatic
-     `GITEA_TOKEN` cannot push packages.
+     Login uses the repo secret `REGISTRY_TOKEN` (an access token with
+     `write:package` scope) because the automatic `GITEA_TOKEN` cannot push
+     packages; the username is just `gitea.actor`.
 - Release: a git tag `vX.Y.Z` produces the versioned image; the Open WebUI
   compose pins that tag. No images are built from branches.