CI: swap order of platforms

GitHub Actions starts the jobs earlier in the list first. This change
has the effect of starting the few longer running jobs (those on ubuntu
20.04) first.

.github/workflows/build.yml

@@ -34,7 +34,7 @@ jobs:
         if [ '${{ github.event_name }}' = 'pull_request' ]; then
           matrix='[ "ubuntu-20.04" ]'
         else
-          matrix='[ "ubuntu-16.04", "ubuntu-18.04", "ubuntu-20.04" ]'
+          matrix='[ "ubuntu-20.04", "ubuntu-18.04", "ubuntu-16.04" ]'
         fi
         echo "::set-output name=matrix::$matrix"