Run lint explicitly on Ubuntu 20.04 to avoid warning

Author: eregon

.github/workflows/test.yml

@@ -103,7 +103,7 @@ jobs:
       run: gem install sassc -N
 
   lint:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v2
     - run: yarn install