hg-5.4: forcing pull for hgext3rd images

We can't currently rebuild everything (no-cache) because of #2,
Without `no-cache`, the `hg clone` layer gets mounted from another
image, that does not have the 5.4 tag.

We could have decided to introduce more build options, but found
simpler to add an intermediate pull to obtain 5.4 images right away.

This can be removed safely once #2 is solved.
1 job for topic/default/hg-5.4 in 22 seconds