nixos.spotifyd: fixed file not found error
nixos.spotifyd: fixed file not found error

When using password_cmd, there's a 'file not found' error due to missing sh binary in path.
For some reason, adding `path = [ "/bin" ]` doesn't fix the issue, but setting `SHELL` does.
Related documentation: https://spotifyd.github.io/spotifyd/config/File.html#shell-used-to-run-commands-indicated-by-password_cmd-or-on_song_changed_hook----omit-in-toc---
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