ci: use unique artifact names

Starting from v4, the actions/upload-artifact action does not allow the
same artifact name uploaded by more than one job.

To overcome that, improve the name of the tito build artifact by:
- mentioning it contains RPMs
- adding the container name that generated it

.github/workflows/ci.yml

@@ -36,6 +36,12 @@ jobs:
         git config --global user.email "[email protected]"
         git config --global user.name "Github Actions"
 
+    - name: Get container name & version
+      id: container-name
+      run: |
+        name=${{ matrix.container }}
+        echo "name=${name##*/}" >> $GITHUB_OUTPUT
+
     - name: Setup EPEL 8
       if: ${{ endsWith(matrix.container, 'stream8') }}
       run: |
@@ -74,6 +80,6 @@ jobs:
     - name: Archive artifacts
       uses: actions/upload-artifact@v4
       with:
-        name: artifacts
+        name: RPMs for ${{ steps.container-name.outputs.name }}
         path: |
           tito/