Fix test-signing workflow to use ubuntu-22.04 for dpkg-sig availability

Author: fdevans

.github/workflows/test-signing.yml

@@ -5,7 +5,7 @@ on: workflow_dispatch
 jobs:
   test-signing:
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     steps:
       - uses: actions/checkout@v3
         with: