mkosi: Only archive outputs in systemd and systemd-stable repositories

Private forks would very quickly reach their quota or spend lots of
money trying to archive all these artifacts, so let's make sure it
only happens on our own repositories.

.github/workflows/mkosi.yml

@@ -188,7 +188,7 @@ jobs:
 
     - name: Archive failed test journals
       uses: actions/upload-artifact@v4
-      if: failure()
+      if: failure() && (github.repository == 'systemd/systemd' || github.repository == 'systemd/systemd-stable')
       with:
         name: ci-mkosi-${{ github.run_id }}-${{ github.run_attempt }}-${{ matrix.distro }}-${{ matrix.release }}-failed-test-journals
         path: |
@@ -197,7 +197,7 @@ jobs:
 
     - name: Archive packages
       uses: actions/upload-artifact@v4
-      if: success() || failure()
+      if: (success() || failure()) && (github.repository == 'systemd/systemd' || github.repository == 'systemd/systemd-stable')
       with:
         name: ci-mkosi-${{ github.run_id }}-${{ github.run_attempt }}-${{ matrix.distro }}-${{ matrix.release }}-packages
         path: |