ci(Makefile): Use system-agnostic path for bash.

[sona-tau]

Currently the SHELL variable in the Makefile uses /bin/bash as a shell. This would normally work except for the users that have the bash executable installed somewhere else on their system. (Example: NixOS users, like me) Using /usr/bin/env bash will ensure that the bash executable is found on any system.

Simply changing this line solved this issue for me.