Sort out some issues with our CI setup

* add missing apt-get update where needed
* move some jobs to run on ubuntu-20.04
* update actions plugins to latest
* speed up the job that runs multiplayer tests

util/ci/common.sh

@@ -17,9 +17,6 @@ install_linux_deps() {
 
 	sudo apt-get update
 	sudo apt-get install -y --no-install-recommends ${pkgs[@]} "$@"
-
-	# workaround for bug with Github Actions' ubuntu-18.04 image
-	sudo apt-get remove -y libgcc-11-dev gcc-11 || :
 }
 
 # Mac OSX build only