tests: Do not assume bash is at /bin/bash

Yay, a NixOS change. See, in a `nix-shell` bash can indeed be available,
but it cannot be made available as /bin/bash. Interestingly enough,
POSIX doesn't even mandate that the classic Bourne shell be at /bin/sh,
but there is a mandate for a standard /usr/bin/env. So let's make my
life easier on this crazy Nix.

Change-Id: I00877d8a6a5083cba6d7e1edd651e27392af5c52
diff --git a/tests/sysrepoctl-manage-module.sh b/tests/sysrepoctl-manage-module.sh
index e9693bb..681dca8 100755
--- a/tests/sysrepoctl-manage-module.sh
+++ b/tests/sysrepoctl-manage-module.sh
@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 set -eux -o pipefail
 shopt -s failglob