[ci] Use known working Ubuntu version

GitHub Actions seems to be moving ubuntu-latest from ubuntu-18.04 to
ubuntu-20.04, but gcovr is broken for us in the new version.

.github/workflows/linux-ci.yml

@@ -8,7 +8,7 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
 
     steps:
     - uses: actions/checkout@v2