Update autobuild.yml

Update to ubuntu-20.04 per actions/runner-images#6002

.github/workflows/autobuild.yml

@@ -25,7 +25,7 @@ on:
 # A workflow run is made up of one or more jobs that can run sequentially or in parallel
 jobs:
   build:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
       # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
       - uses: actions/checkout@v2