ci: supersede in-progress runs on push to main (#1167)

The concurrency group fell back to github.run_id when github.head_ref
was empty, which is every push to main. A unique run_id per run meant no
two runs ever shared a group, so cancel-in-progress never fired on main.
A burst of merges then stacked CI and Docker runs that all ran to
completion, saturating the shared (especially macOS) runner pool and
pushing wall-clock into the hours through queue contention.

Fall back to github.ref instead, so every push to a branch shares one
group and the newer run supersedes the older one. Pull requests are
unchanged: head_ref is still set there and keys the group by source
branch.

Author: leolara

.github/workflows/ci.yml

@@ -11,7 +11,7 @@ permissions:
   contents: read
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  group: ${{ github.workflow }}-${{ github.head_ref || github.ref }}
   cancel-in-progress: true
 
 jobs:

.github/workflows/docker.yml

@@ -11,7 +11,7 @@ permissions:
   packages: write
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  group: ${{ github.workflow }}-${{ github.head_ref || github.ref }}
   cancel-in-progress: true
 
 jobs: