common: Fix bash shell script invocation

`#!/usr/bin/env bash` should be used to avoid hard paths.

To address:

  dockcross-manylinux-x64: bad interpreter: /bin/bash^M: no such file or directory

README.rst

@@ -302,7 +302,7 @@ Per-project dockcross configuration
 If a shell script named ``.dockcross`` is found in the current directory where
 the dockcross script is started, it is executed before the dockcross script
 ``command`` argument.  The shell script is expected to have a shebang like
-``#!/bin/bash``.
+``#!/usr/bin/env bash``.
 
 For example, commands like ``git config --global advice.detachedHead false`` can
 be added to this script.