RFC: Configurable placing of the ~/.guix-home symlink

[EuAndreh]

Hi Guix!

When trying to get a clean $HOME, I see that the ~/.guix-home path is fixed in
the code under gnu/home/*,
usually via (string-append (GETENV "HOME") "/.guix-home").

I want to propose a configurable path to ~/.guix-home, and allow this to be
chosen in the (home-environment ...) declaration.  It would expose a
GUIX_HOME_PATH_ENVIRONMENT variable[0] so that one could dynamically retrieve
the path to it, if needed in scripts or similar.  The implementation would also
need to be able to handle changes in the (home-environment ...) declaration,
and know when to remove the old symlink from ~/.guix-home to the new path,
similar to what the gnu/home/services/symlink-manager.scm does nowadays.

How do you feel about this?  What do you think?  Is there a shortcoming,
pitfalls, limitations that this approach entails?

I'm willing do work on the implementation and on tests where applicable, and
send the patches (eventually when I get this done =p).  I wanted to raise a
discussion before jumping into the code, even to get input and see how the
community feels about this.

WDYT?


[0]: I'm not picking this name, I just made up to make this more concrete.  I'd
     look at the existing variable declarations, such as those exposed by
     `guix shell` to make it consistent with them.