Subject: guix-install.sh: uses /bin/bash shebang

From: Claes Wallin (韋嘉誠) <[email-masked]>
Date: Fri, 1 Feb 2019 16:35:56 +0100

This means ./guix-install.sh doesn't work on e.g. NixOS. :-)

Please use /usr/bin/env bash.

The manual workaround is obvious, but this is a thousand-papercuts bug.