Merge branch 'backport/jx/ci-ubuntu-fix' into maint-2.30

Adjust the GitHub CI to newer ubuntu release. * jx/ci-ubuntu-fix: github-actions: run gcc-8 on ubuntu-20.04 image ci: install python on ubuntu ci: use the same version of p4 on both Linux and macOS ci: remove the pipe after "p4 -V" to catch errors