tests/module: nix-ify

tests/module: nix-ify

Use "#!/usr/bin/env bash" instead of "#!/bin/bash". This is necessary
for nix environments as they only provide /usr/bin/env at the standard
location.

Signed-off-by: Joel Granados <joel.granados@kernel.org>
Acked-by: Luis Chamberlain <mcgrof@kernel.org>
Link: https://lore.kernel.org/r/20250122-jag-nix-ify-v1-1-addb3170f93c@kernel.org
Signed-off-by: Petr Pavlu <petr.pavlu@suse.com>