CI: force bubblewrap tests to run on whitewhale.

nested user namespaces are not enabled on the other runners.

.gitlab-ci.yml

@@ -48,6 +48,8 @@ tests:debian:
 
 tests:debian_with_bubblewrap:
   stage: test
+  tags:
+    - whitewhale
   script:
   - apt-get -qqy update
   - apt-get -qqy install --no-install-recommends python3-mutagen python3-gi-cairo gir1.2-poppler-0.18 gir1.2-gdkpixbuf-2.0 libimage-exiftool-perl python3-coverage ffmpeg bubblewrap