Use environment variables to provide mirror URLs to checkout
sources from Git. Each component has a corresponding env var
that will be used if it's present: <component>_GIT_MIRROR.
Note that '<component>' should be upper case, e.g. GLIBC.
Co-authored-by: Carlos O'Donell <carlos@redhat.com>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>