gitlab-ci: replace '$os' tags with '$os-x86_64' on 3.25 release branch
The latter tags are now available on our CI runners to more explicitly
distinguish linux-x86_64
from linux-aarch64
and to distinguish
{macos,windows}-x86_64
from {macos,windows}-arm64
.
This backports the tag updates from !7973 (merged) to the 3.25 release branch.
Backport: release:HEAD^2
Edited by Brad King