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
```
When working on a new build script, I tripped over the `-?` question
mark help parameter here, as it needs to be escaped. Opted to fix all
the instances in our scripts, although AFAICT there would not have been
current bugs arising from this, as there were no conflicting one
character options lower in the case list.
