chore(common): use /usr/bin/env bash

Fixes up scripts (except under /linux) to use `#!/usr/bin/env bash`
instead of `#!/bin/bash` or `#!/bin/sh` so that we don't end up with
the ancient version of bash supplied with macOS.

This became urgent with this PR, because of bash-4.xisms in
build-utils.sh, for example on line 572:

```
if [[ -v _builder_params[$e] ]]; then
```