CI building unikernels should use correct branch

[hannesm]

At the moment, each unikernel adds a "pin-depend" on the miragevpn#main repository for the miragevpn package.

This makes the cirrus CI testing the unikernels with the main branch of miragevpn instead of the current branch.

We should change this, maybe we can have a script after depend to "rm -rf duniverse/miragevpn && ln -s duniverse/miragevpn` to always use the right branch.