feat: move to more robust solution than time-machine

The problem with time-machine is that the guix
derivation might need to be recomputed and rebuilt.
This can happen because the underlying guix version used
to build it can differ. The solution to this can be
to keep the resulting guix in a folder, and use it out of
that folder, instead of recomputing it every time.

This can still mean that there will be need for
duplicit recomputation between building the guix
and pulling it into user profile. I don't know what
a good solution to this is, yet.