Switch the runner to ubuntu-20.04

Author: mamercad

.github/workflows/build.yaml

@@ -19,7 +19,7 @@ on:
 jobs:
   integration:
     name: 'integration - ${{matrix.name}}'
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       fail-fast: false
       matrix:
@@ -107,7 +107,7 @@ jobs:
     if: always()
     needs:
       - integration
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - name: Workflow conclusion
         # this step creates an environment variable WORKFLOW_CONCLUSION and is the most reliable way to check the status of previous jobs