[.github] Add shared action `upload-job-summary`

replace duplicate code like this in several workflows:

```yaml
- if: ${{ always() && steps.generate-job-summary.outputs.summary }}
name: Set job-summary artifact
uses: actions/upload-artifact@v7
with:
name: job-summary
path: ${{ steps.generate-job-summary.outputs.summary }}
# If the file doesn't exist, just don't add the artifact
if-no-files-found: ignore
```

with a call to a reusable action:

```yaml
- if: ${{ always() && steps.generate-job-summary.outputs.summary }}
name: Set job-summary artifact
uses: ./.github/actions/upload-job-summary
with:
path: ${{ steps.generate-job-summary.outputs.summary }}
```

This removes a few lines of duplicate code (`name`, `if-no-files-found`). But more importantly, it gives us a reusable action we can update in a single place, for example if we want to start using unzipped uploads.