workflows/eval: upload maintainer list of all packages as artifact

This creates a simple mapping of all packages to github ids of all their
maintainers. This is uploaded as an artifact and is then available for
download on the latest commit of each branch with a merge queue. This
will allow scheduled jobs to use this information for setting
maintainer-related labels, to request reviewers and to implement the
merge-bot.

The advantage over querying this information directly via Nix in each
case: The scheduled job does not need to install Nix and does not need
to checkout the target branch.

Compared to obtaining the maintainer information just for a single PR
during Eval, this will allow retroactively changing maintainers for a
package: For example, it allows to request a new maintainer as reviewer
for a PR that was created before they became maintainer, but is still
open - and similarly for maintainer labels and merge-bot rights.

None of these extensions are implemented by this PR, yet.

.github/workflows/eval.yml

@@ -194,6 +194,12 @@ jobs:
             --arg diffDir ./diff \
             --out-link combined
 
+      - name: Upload the maintainer list
+        uses: actions/upload-artifact@330a01c490aca151604b8cf639adc76d48f6c5d4 # v5.0.0
+        with:
+          name: ${{ inputs.artifact-prefix }}maintainers
+          path: combined/maintainers.json
+
       - name: Compare against the target branch
         env:
           AUTHOR_ID: ${{ github.event.pull_request.user.id }}