ci: fix concurrency group causing master badge to show cancelled

github.head_ref is only set for pull_request events; it is empty for
push-to-master and tag events. When two push events land in quick
succession (e.g. merge commit + release tag), both go into the same
unnamed group 'test-' and the second cancels the first, causing the
master badge to show 'cancelled'.

Using `github.head_ref || github.ref` gives each branch/tag its own
concurrency group while keeping the cancel-in-progress behaviour for
PRs (where head_ref is 'refs/pull/NNN/merge', unique per PR).

Author: utiberious

.github/workflows/ci.yml

@@ -8,7 +8,7 @@ on:
   pull_request:
 
 concurrency:
-  group: test-${{ github.head_ref }}
+  group: test-${{ github.head_ref || github.ref }}
   cancel-in-progress: true
 
 env: