bug#66168: zsh home service breaks guix shell via ssh

[Saku Laesvuori via Bug reports for GNU Guix <bug-guix@gnu.org>]

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On Sat, Sep 23, 2023 at 12:19:10PM +0200, Malte Frank Gerdes wrote:
> I noticed that after connecting via ssh the profile that gets created
> when doing `guix shell' is not sourced, therefore the environment
> variables are not correct.  This seems to be because guix home inserts
> "[ -n \"$SSH_CLIENT\" ] && source /etc/profile" into the zshenv file.  I
> assume this is meant to ensure that /etc/profile is sourced because ssh
> does not start a login shell?

That is correct, it was added in <https://issues.guix.gnu.org/64765>
because commands like `ssh host -- cmd` (and commands depending on ssh
such as `git pull/push`) failed. 


> Anyway, removing that line makes everything work normally.  So am i
> doing it wrong or is this actually unnecessary?

I can reproduce this:

```
$ ssh localhost
$ guix shell gnuplot
$ gnuplot
zsh: command not found: gnuplot
```

However (still within the ssh session)

```
$ guix shell gnuplot -- gnuplot
[gnuplot opens as expected]
```

and weirdly enough

```
$ guix shell --check gnuplot
guix shell: checking the environment variables visible from shell '/gnu/store/i4rj0banq044423if5xwb3df723g14r9-zsh-5.9/bin/zsh'...
guix shell: All is good!  The shell gets correct environment variables.
$ gnuplot
zsh: command not found: gnuplot
```

I don't really know what is happening here. Why does it work when
running the command directly and why does `guix shell --check` claim
that everything is correct when clearly something is wrong? If I
understand correctly `.zshenv` should be sourced in all these cases but
for some reason the problems only occur in some of them.

Any ideas?

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