workflows/{pr,push}: init

Those two workflows bundle all the main jobs in two event-specific
wrapper workflows. This enables us to do two things later on:
- Synchronize the merge commits between most of the jobs run in a PR.
- Create a single "required" job to be targeted by GitHub's "required
status checks to pass" feature.

(cherry picked from commit 959eed1f2a)

.github/workflows/check.yml

@@ -1,14 +1,7 @@
 name: Check
 
 on:
-  pull_request:
-    paths:
-      - .github/workflows/check.yml
-  pull_request_target:
-
-concurrency:
-  group: check-${{ github.workflow }}-${{ github.event_name }}-${{ github.event.pull_request.number || github.run_id }}
-  cancel-in-progress: true
+  workflow_call:
 
 permissions: {}