Change linux version to ubuntu-20.04

.github/workflows/build.yaml

@@ -7,7 +7,7 @@ on:
 jobs:
   build-linux:
     name: Build Linux
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - name: Fix up git URLs
         run: echo -e '[url "https://github.com/"]\n  insteadOf = "git@github.com:"' >> ~/.gitconfig