This avoids the uncontrollable nature of fetching the tarball as part
of the evaluation. Instead the user can decide when to update and also
perform rollbacks, if necessary.
In particular, don't bother attempting to do substitution of the home
files and home generation derivations since these rarely, if ever,
could be substituted.
Fixes#330
This changes the installation command from
nix-shell $HM_PATH -A install --run 'home-manager switch'
to
nix-shell $HM_PATH -A install
The added shell hook will print some useful information and run
`home-manager switch`.
