Issue skupperproject#1240: Use GCC 13 in Ubuntu Github Actions jobs, …

…hoping to avoid kills

This PR follows a similar attempt at solving the issue by upgrading GCC
attempted at VROOM-Project/vroom#1002

.github/workflows/build.yaml

@@ -54,6 +54,8 @@ jobs:
           - ${{ github.event.inputs.protonBranch || 'main' }}
           - 0.39.0
     env:
+      CC: 'gcc-13'
+      CXX: 'g++-13'
       BuildType: ${{matrix.buildType}}
       ProtonBuildDir: ${{github.workspace}}/qpid-proton/build
       RouterBuildDir: ${{github.workspace}}/skupper-router/build
@@ -211,6 +213,8 @@ jobs:
         shard: [1, 2]
         shards: [2]
     env:
+      CC: 'gcc-13'
+      CXX: 'g++-13'
       BuildType: ${{matrix.buildType}}
       ProtonBuildDir: ${{github.workspace}}/qpid-proton/build
       RouterBuildDir: ${{github.workspace}}/skupper-router/build