github/workflows/ci: Update to Ubuntu 20.04

This was promoted to `ubuntu-latest`.

Author: zaeleus

.github/workflows/ci.yml

@@ -4,7 +4,7 @@ on: [push, pull_request]
 
 jobs:
   test:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v2
       - run: cargo test