In bug 1596135 comment 9, :jlund called out a release blocker because the
snap-repackage task cannot download a tarball of the source code via hg.mozilla.org. I hit the same issue, when I wanted to test bug 1591751 out on try.
At the beginning, I started to look into :sheehan's recommendation (bug 1596135 comment 11), but I later realized there's a smarter fix to make. Nowadays, the firefox-snap docker image is rebuilt whenever the content of  changes. This folder contains everything the repackage task needs, but interestingly, it's not baked in the docker image. We keep downloading these files from hg.mozilla.org directly. I traced back the origin of this behavior and it was first introduced in bug 1297519, that is to say, at the very beginning of the Snap project. At that time, we didn't have docker images automatically built in tree, we pulled images from dockerhub. Docker images in tree were made possible by bug 1343107.
Therefore, we've carried over this old behavior for more than 3 years. We can now use better tools.