dist/freedoom: use “/usr/bin/env” for finding bash

Not all systems (FreeBSD or Mac OS X, as examples) have a bash
installed at /bin/bash.  Using /usr/bin/env is a OS-portable way to
invoke a script.

dist/freedoom

@@ -1,4 +1,4 @@
-#!/bin/bash -
+#!/usr/bin/env bash
 
 # These ports should be ordered by ease-of-use, of which menu
 # simplicity and support for high-resolution take high priority.