Use VM GitLab runner

The Docker-based runners do not support IPv6 due to a bug in Docker [1].
Future changes will require IPv6 support to run the unit tests, so use
the VM-based runners instead.  As per Frédéric Pierre, this will not
slow down CI.

[1]: moby/moby#33099

.gitlab-ci.yml

@@ -9,8 +9,7 @@ include:
 .qrexec_checks:
   stage: checks
   tags:
-    - short-living-job
-    - docker
+    - vm
   before_script: &before-script
     - "PATH=$PATH:$HOME/.local/bin"
     - sudo dnf install -y python3-pip python3-gobject gtk3 python3-pytest