base: Rename the recipe to rootfs.yaml

Calling the recipe `apertis.yaml` is annoying for downstreams which have
to either keep the misleading name or rename it and deal with issues
when pulling updates.

Let's use a slightly more descritive name that does not encode the
name of the OS.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

.gitlab-ci.yml

@@ -49,7 +49,7 @@ build-base-rootfs:
   script:
     - >
       cd base &&
-      debos -t suite:$RELEASE -t stable:$STABLE $DISTRO.yaml
+      debos -t suite:$RELEASE -t stable:$STABLE rootfs.yaml
 
 .build-docker-image:
   image: