ci: Don't inherit variables on downstream pipeline

This commit was cherry-picked from !6608 (merged) which alerted us to the issue.

The inherit:variables option is used to control whether variables set in a parent CI configuration is inherited by its children. In our case, the parent is the Gitaly CI pipeline and the child is the omnibus-gitlab-mirror downstream pipeline, which we trigger here.

Disable variable inheritance as it appears to cause a compilation error in the Omnibus pipeline when it attempts to build the Spamcheck binary. Spamcheck is written in Go, so it's possible that we're setting some specific toolchain options that conflict with its build process.

Other projects that trigger this downstream, such as gitlab-shell, also explicitly disable variable inheritance. See https://gitlab.com/gitlab-org/gitlab-shell/-/blob/03bb2b21172254b42ea5bbed0b6de9733b7dadf1/.gitlab-ci.yml#L195-196