gitlab: allow overriding name of the upstream repository

gitlab: allow overriding name of the upstream repository

The CI rules have special logic for what happens in upstream. To enable
contributors who modify CI rules to test this logic, however, they need
to be able to override which repo is considered upstream. This
introduces the 'QEMU_CI_UPSTREAM' variable

  git push gitlab <branch> -o ci.variable=QEMU_CI_UPSTREAM=berrange

to make it look as if my namespace is the actual upstream. Namespace in
this context refers to the path fragment in gitlab URLs that is above
the repository. Typically this will be the contributor's gitlab login
name.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20230608164018.2520330-3-berrange@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>