Switch from Ubuntu 16.04 to 18.04

This is only for GitHub Actions artifacts (due to actions/virtual-environments#3287),
thus official releases will hopefully still based on 16.04.

.github/workflows/main.yml

@@ -3,7 +3,7 @@ on: [push, pull_request]
 
 jobs:
   linux-os:
-    runs-on: ubuntu-16.04
+    runs-on: ubuntu-18.04
     steps:
     - name: Update APT
       run: sudo apt-get update