test:update step fails on forks

From https://gitlab.com/niskhakova/gitlab-development-kit/-/jobs/221741934, the self-update step fails because the commit isn't available:

cd /home/gdk/gitlab-development-kit && \
	git stash && \
	git checkout master && \
	git fetch && \
	support/self-update-git-worktree
No local changes to save
Already on 'master'
Your branch is up to date with 'origin/master'.
fatal: reference is not a tree: 9fc4df6012874dd5f35d0e19532065cafd03cb0a
Makefile:241: recipe for target 'self-update' failed
make: *** [self-update] Error 128