micropython: unhardcode /bin/bash

/bin/bash does not exist on odd unixen (BSDs, NixOS, ...). /usr/bin/env is the current de facto standard
to get bash somewhat universally.

lib/micropython/gen-frozen.sh

-#!/bin/bash
+#!/usr/bin/env bash
 set -e
 
 PYTHON="$1"

lib/micropython/gen-mpy-cross.sh

-#!/bin/bash
+#!/usr/bin/env bash
 set -e
 
 SOURCE_DIR="$1"

lib/micropython/gen-qstr.sh

-#!/bin/bash
+#!/usr/bin/env bash
 set -e
 
 PYTHON="$1"