ci: export XDG_RUNTIME_DIR when running the tests

Recent changes make some tests start an internal pipewire server,
which requires XDG_RUNTIME_DIR to create the socket file
2 jobs for master in 2 minutes and 8 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Container
passed build-container #1143454

00:00:27

 
  Build
passed build #1143455

00:01:41