tools: Use env in shebangs

Having perl and bash in /usr/bin/ is less common than having them on PATH
(e.g. Nix does not have them there).

app/tests/create_test_env.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # Copy tests resources
 for dir in files gimpdir gimpdir-empty; do

tools/gimp-mkenums

@@ -1,4 +1,6 @@
-#!/usr/bin/perl -w
+#!/usr/bin/env perl
+
+use warnings;
 
 # This is gimp-mkenums, a perl script based on glib-mkenums.
 # It can be used just like glib-mkenums but offers one extra