[CI] Give push events a unique concurrency group

The concurrency group keys on github.event.pull_request.number, which is
set only for pull_request events. On a push to master that expression is
empty, so every master push collapses into one group and, with
cancel-in-progress, each new push cancels the previous run mid-flight.

The self-hosted coverage row (wake-on-lan, then a CUDA + Enzyme +
sanitizer build) is by far the slowest job, so on master it is almost
always cancelled before the codecov upload step runs. Master never gets a
base coverage report, and every PR's codecov/project status -- which uses
`target: auto` and needs the base coverage -- stalls at "Expected --
Waiting" indefinitely. codecov/patch is unaffected: it needs only the
PR's own report.

Fall back to github.sha when there is no PR number, so each pushed commit
gets its own group and push runs stop cancelling each other.

Author: vgvassilev

.github/workflows/ci.yml

@@ -8,7 +8,7 @@ on:
       - master
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.event.pull_request.number }}
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.sha }}
   cancel-in-progress: true
 
 jobs: