Switch ci to ubuntu 20.04

Author: mgautierfr

.github/workflows/pull_request.yml

@@ -9,7 +9,7 @@ jobs:
 
   build_job:
     name: Build Pull Request
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
 
     steps:
       - uses: actions/checkout@v1