Run CI on ubuntu 20.04 (#1199)

Author: marmichalski

.github/workflows/build-vcpkg.yml

@@ -45,7 +45,7 @@ jobs:
               glew luajit libogg libvorbis openal-soft opengl openssl physfs zlib
           - name: ubuntu-gcc
             os: ubuntu
-            os-version: 18.04
+            os-version: 20.04
             cxx: g++
             cc: gcc
             triplet: x64-linux