chore(ci): explicit concurrency groups now the shared runner holds 2 slots

Capacity 1 on the ovh-main runner was the implicit mutex serializing every
workflow; sow-platform#189 raises it to 2. Side-effectful workflows get a
serialized group; PR-triggered checks get a per-ref group with cancel so a
superseded head frees its slot.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.gitea/workflows/build-binaries.yml

@@ -11,6 +11,12 @@ permissions:
   code: read
   releases: write
 
+# The shared runner runs 2 jobs at once now (sow-platform#189); capacity 1 was
+# the implicit mutex. Serialize this workflow explicitly; never cancel mid-run.
+concurrency:
+  group: build-binaries
+  cancel-in-progress: false
+
 jobs:
   build-binaries:
     runs-on: nix-docker