bug#46782: guix environment --expose options cannot be layered onto $PWD

[Maxim Cournoyer <maxim.cournoyer@gmail.com>]

Hello Guix!

I noticed, while trying to expose the SSL environment variable inside a
container and doing things such as:

--8<---------------cut here---------------start------------->8---
guix environment --container --network -E SSL --expose=$SSL_CERT_FILE \
     --expose=$SSL_CERT_DIR --ad-hoc [...]
--8<---------------cut here---------------end--------------->8---

that it wasn't possible to expose things onto the implicitly mounted
current working directory.  No error message would be shown, yet the
--expose arguments wouldn't go through unless using --no-cwd.

Here's a minimal example:

--8<---------------cut here---------------start------------->8---
$ guix environment -C --expose=/tmp=$PWD/tmp \
  --ad-hoc bash coreutils -- bash -c 'stat $PWD/tmp'
stat: cannot statx '/var/lib/jenkins/tmp': No such file or directory
--8<---------------cut here---------------end--------------->8---

but,

--8<---------------cut here---------------start------------->8---
$ guix environment --no-cwd -C --expose=/tmp=$PWD/tmp \
  --ad-hoc bash coreutils -- bash -c 'stat $PWD/tmp'
  File: /var/lib/jenkins/tmp
  Size: 73728           Blocks: 152        IO Block: 4096   directory
Device: fd00h/64768d    Inode: 1966081     Links: 397
Access: (1777/drwxrwxrwt)  Uid: (65534/ UNKNOWN)   Gid: (65534/overflow)
Access: 2021-02-25 20:40:08.534757708 +0000
Modify: 2021-02-25 21:10:49.205636074 +0000
Change: 2021-02-25 21:10:49.205636074 +0000
 Birth: 2016-12-07 20:43:16.029221570 +0000
--8<---------------cut here---------------end--------------->8---

succeeds.

I can't think of a technical reason why layering bind mounts wouldn't
work; in fact it already appear to work for multiple --expose options
touching the same hierarchy of files.

If there's a technical reason this cannot/shouldn't happen, we should
warn the user about it.

Thanks,

Maxim