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

manylinux-common/install-python-packages.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 for PIP in /opt/python/*/bin/pip; do
   $PIP install --disable-pip-version-check --upgrade pip

manylinux-common/pre_exec.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 for DIR in /opt/python/*/lib/python*/site-packages; do
   chown -R $BUILDER_UID:$BUILDER_GID $DIR