CI: Use staging repo to check Python packages reverse dependencies.

.github/workflows/main.yml

@@ -117,6 +117,14 @@ jobs:
           msystem: MINGW64
           install: mingw-w64-x86_64-python-pip
           release: false # since we aren't building anything
+      - name: Add staging repo
+        shell: msys2 {0}
+        run: |
+          sed -i '1s|^|[staging]\nServer = https://repo.msys2.org/staging/\nSigLevel = Never\n|' /etc/pacman.conf
+      - name: Update using staging
+        run: |
+          msys2 -c 'pacman --noconfirm -Suuy'
+          msys2 -c 'pacman --noconfirm -Suu'
       - name: Run Checks
         if: steps.check-run.outputs.run == 'true'
         shell: msys2 {0}