pythongh-134309: Add github.actor to the GitHub Actions concurrency key (python#134310)

When inexperienced users create a PR from their default branch, all of the concurrency keys
collide as there is no namespacing. This becomes an issue at events with many new contributors,
where workflow runs are cancelled on other pull requests.
Disambiguate by adding the username of the relevant 'actor' to the concurrency key.

Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>
Co-authored-by: Sviatoslav Sydorenko <sviat@redhat.com>
Co-authored-by: C.A.M. Gerlach <CAM.Gerlach@Gerlach.CAM>
Authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>

Author: 4 people

.github/workflows/build.yml

@@ -15,7 +15,13 @@ permissions:
   contents: read
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}-reusable
+  # https://docs.github.com/en/actions/writing-workflows/workflow-syntax-for-github-actions#concurrency
+  # 'group' must be a key uniquely representing a PR or push event.
+  # github.workflow is the workflow name
+  # github.actor is the user invoking the workflow
+  # github.head_ref is the source branch of the PR or otherwise blank
+  # github.run_id is a unique number for the current run
+  group: ${{ github.workflow }}-${{ github.actor }}-${{ github.head_ref || github.run_id }}
   cancel-in-progress: true
 
 env: