Nix need to have its guile path envvar set

[rixed]

Namely:

• GUILE_LOAD_PATH=$out/share/guile/site
• GUILE_LOAD_COMPILED_PATH=$out/lib/guile/ccache

We could install a wrapper around junkie that set both but then plugins autoload wouldn't work.
So better add them to user defined ones but what's the nix way of doing so?