ci: stop build.yml from also running on tag pushes

A bare `push:` trigger matches every ref push, tags included -- so
tagging a release triggered this workflow's full 3-platform build
(Windows and all) at the same time as release.yml's own, both
competing for the Windows runner's capacity:1 and roughly doubling
Windows CI time for every tag push. Scope this workflow's push trigger
to branches only; release.yml already covers this same build (plus
packaging) on every `v*` tag.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RL8abRmgFXkVASHkkqiJbE

.gitea/workflows/build.yml

@@ -31,6 +31,15 @@ name: Build
 
 on:
   push:
+    # Excludes tag pushes -- a bare `push:` matches every ref push, tags
+    # included, which meant tagging a release triggered THIS workflow's full
+    # 3-platform build (Windows and all) at the same time as
+    # release.yml's own -- two full Windows builds serialized behind the
+    # runner's capacity:1, for one tag push. release.yml already covers
+    # exactly this build (plus packaging) on every `v*` tag; this workflow's
+    # job is ordinary commits.
+    branches:
+      - "**"
   pull_request:
 
 jobs: