ci: Check out virtme-ng tag instead of branch

The upstream branch is gone, so use the corresponding tag instead.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3081>

.gitlab-ci.yml

@@ -143,7 +143,7 @@ variables:
         git clone https://github.com/arighi/virtme-ng.git
         cd virtme-ng
         git fetch --tags
-        git checkout upstream/1.8
+        git checkout v1.8
         ./setup.py install --prefix=/usr
         cd ..
         rm -rf virtme-ng