fix(ci): Explicitly request Ubuntu 18.04

Author: emk

.github/workflows/ci.yml

@@ -17,7 +17,7 @@ jobs:
   # This workflow contains a single job called "build"
   build:
     # The type of runner that the job will run on
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
 
     # Steps represent a sequence of tasks that will be executed as part of the job
     steps: