Build on Ubuntu 20.04 too

Author: eregon

.github/workflows/build.yml

@@ -50,7 +50,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ ubuntu-16.04, ubuntu-18.04, macos-latest, windows-latest ]
+        os: [ ubuntu-16.04, ubuntu-18.04, ubuntu-20.04, macos-latest, windows-latest ]
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/download-artifact@v1