fix: build the runner with a schema that accepts Gitea 28's matrix jobs

Gitea 28 adds job-index and job-total under strategy in the workflow it
sends for a matrix job, and forgejo-runner's schema refuses them, so
every matrix job dies before its first step (zegit image-gitea.yml run
23992, the v0.1.52 tag build). No forgejo-runner release accepts them
(checked through 13.2.1). Build the runner from v12.11.1 at its commit
with a one-file schema patch and two tests, one of them the real payload
of that failed job; the build runs them. Bases are pinned by digest.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_018wWyCiV27KmNSauYTdLyp7

.github/workflows/build-image-gitea.yml

@@ -33,6 +33,7 @@ on:
     branches: [master]
     paths:
       - 'Dockerfile'
+      - 'patches/**'
       - '.github/workflows/build-image-gitea.yml'
 
 env: