workflows/{labels,reviewers}: fix concurrency groups for nested workflows

This didn't work as intended. When a workflow is run with
`workflow_call`, it will have `github.workflow` set to the *parent*
workflow. So the `caller` input that we passed, resulted in this
concurrency key:

```
Eval-Eval-...
```

But that's bad, because the labels and reviewers workflows will cancel
each other!

What we actually want is this:
- Label and Reviewers workflow should have different groups.
- Reviewers called via Eval and called directly via undraft should have
*different* groups.

We can't use the default condition we use everywhere else, because
`github.workflow` is the same for Label and Reviewers. Thus, we hardcode
the workflow's name as well. This essentially means we have this as a
key:

```
<name-of-running-workflow>-<name-of-triggering-workflow>-<name-of-event>-<name-of-head-branch>
```

This should do what we want.

Since workflows can be made reusable workflows later on, we add those
hardcoded names to *all* concurrency groups. This avoids copy&paste
errors later on.

(cherry picked from commit 6793e238fa)

.github/workflows/build.yml

@@ -7,7 +7,7 @@ on:
   pull_request_target:
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.event_name }}-${{ github.event.pull_request.number || github.run_id }}
+  group: build-${{ github.workflow }}-${{ github.event_name }}-${{ github.event.pull_request.number || github.run_id }}
   cancel-in-progress: true
 
 permissions: {}