runner: add python or platform-python to all build roots

Required for running osbuild modules.
In Fedora, so far, python3 was always pulled by some other build-root
dependency.  We should add it explicitly since it's required by all our
runners.
For RHEL and CentOS, the requirement is platform-python.

For RHEL 8 and CentOS 8, we also need to explicitly add python36 and for
RHEL 9 and CentOS 9 python3 because it's used to run the stages in
osbuild [1].

[1] ea8261cad6/runners/org.osbuild.rhel82 (L61)

internal/runner/linux.go

@@ -11,5 +11,6 @@ func (p *Linux) GetBuildPackages() []string {
 	return []string{
 		"glibc",   // ldconfig
 		"systemd", // systemd-tmpfiles and systemd-sysusers
+		"python3", // osbuild
 	}
 }