zz-firecfg.py.sh: use portable shell features (enables bash/zsh/others)

[utrack]

LLM-generated, human-tested :)

This commit makes it work in zsh and ksh, and bash works as it did before. I didn't test any other shells, but that's already better :)

[rusty-snake]

Ok, but why do we need this?

[utrack]

The current version silently exits if the shell doesn't have $BASH_VERSION (i.e. not bash, I guess).

On zsh, source /etc/profile.d/zz-firecfg.py.sh (assuming Arch AUR package is installed) does nothing - the paths don't change.

[rusty-snake]

Ahh yeah right, there is also an entrypoint via /etc/profile.d. And some user break things by changing login shell.

[utrack]

Well yeah, that and the fact that 'sh' is a valid login shell too...
As far as I see, every other profile script on my machine is in portable sh syntax.