[bug#30256] [PATCH 3/3] scripts: environment: Add --no-cwd.

[Carl Dong]

Hi Mike and Ludo!

I believe I've found a solution to the problem that Ludo was encountering. The
reason why Ludo was having trouble was because when a user specifies `--user`,
we rewrite the targets of our filesystem mappings so that every instance of
`$HOME` (as seen ouside the container) becomes `/home/$USER`. Since this applied
to all filesystem mappings, it included our filesystem mappings for inputs too.
However, our symlinks were not updated.

My change makes it so that we _only_ update the mappings that are either
user-specified, or cwd (if applicable). This solves Ludo's problems.

Here's the patch, let me know if it looks good:
https://github.com/dongcarl/guix/compare/8e92d5465fc154fed5d06f7e4a64d7dcccded74d...2019-06-env-no-cwd-fix.patch

Cheers,
Carl Dong