ci: fix cancelled jobs in CI merge queue

It is hard to find proper documentation for that and what fails here doesn't fail in
other projects. In a GitHub merge queue, each job is created twice by default (as we
have `on merge_group and on pull_request` in the CI file):

- for the PR/push itself
- for the merge queue

Currently, sometime our merge queue fails because some of these jobs are cancelled:

```
Canceling since a higher priority waiting request for Build-refs/heads/gh-readonly-queue/main/pr-89-b4aed85cf70ae7e2df098e42c352c3dd62ef9173 exists
```

To prevent cancellation, I try to switch to more distinct concurrency groups. I didn't
find proper documentation or best practices, but with some research, I came up with this
solution.

On-behalf-of: SAP philipp.schuster@sap.com
Signed-off-by: Philipp Schuster <philipp.schuster@cyberus-technology.de>

Author: phip1611

.github/workflows/build.yml

@@ -6,8 +6,10 @@ on:
   push:
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.ref }}
-  cancel-in-progress: true
+  # Special care needs to be taken here to prevent cancelled runs in the GitHub
+  # merge queue.
+  group: ${{ github.workflow }}-${{ github.ref }}-${{ github.event_name }}
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
 
 jobs:
   checks: