ci: Allow concurrency outside of pull requests

When multiple merges to `master` happen before a CI run completes, the
in-progress job is getting canceled. Fix this by using the commit sha
for the group key if a pull request number is not available, rather than
`github.ref` (which is always `refs/head/master` after merge). This
should prevent jobs running on previous commits from getting cancelled,
while still ensuring there is only ever one active run per pull request.

Author: tgross35

.github/workflows/main.yaml

@@ -5,7 +5,7 @@ on:
 
 concurrency:
   # Make sure that new pushes cancel running jobs
-  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.sha }}
   cancel-in-progress: true
 
 env: