fix: prevent host-spawn from consuming profile stdin

[hypengw]

Fix distrobox_profile.sh and fish profile generation so host-spawn does not consume the profile shell's stdin.

When a login shell is started with script input from stdin, for example:

echo 'echo xxx;' | bash -l

The generated profile can run host-spawn while inheriting that same stdin. This may consume the remaining script input before the login shell executes it.

This redirects host-spawn calls to use /dev/null .

[balanza]

Hi @hypengw, thanks for the contribution. I am not sure I understand correctly what is the scenario that you are addressing. Can you provide more instructions to replicate your setup?

Thanks