Don't cancel GHA runs on subsequent push events (Qiskit#9917)

With our switch to using the GitHub merge queue with speculative
merging, it's now possible for us to have two push events to a protected
branch (like `main`) within a couple of minutes.  If this happens, the
eager cancellation rules we have set for the coverage and Neko runs in
GHA trigger, causing a spurious build failure to appear in the commit
logs, and for the coverage history to be lost for one commit.

Instead, we can set the cancel-on-update behaviour to only trigger on PR
sync events, not branch-push events.

.github/workflows/coverage.yml

@@ -5,7 +5,9 @@ on:
     branches: ['main']
 concurrency:
   group: ${{ github.repository }}-${{ github.ref }}-${{ github.head_ref }}-${{ github.workflow }}
-  cancel-in-progress: true
+  # Only cancel in PR mode.  In push mode, don't cancel so we don't see spurious test "failures",
+  # and we get coverage reports on Coveralls for every push.
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
 jobs:
   coverage:
     name: Coverage

.github/workflows/neko.yml

@@ -5,7 +5,8 @@ on:
     branches: ['main', 'stable/*']
 concurrency:
   group: ${{ github.repository }}-${{ github.ref }}-${{ github.head_ref }}-${{ github.workflow }}
-  cancel-in-progress: true
+  # Only cancel in PR mode.  In push mode, don't cancel so we don't see spurious test "failures".
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
 jobs:
   neko:
     name: Qiskit Neko Integration Tests