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
```
Fixes#5700.
Adds support for arm64 (M1) to textinputsource in the Keyman app. All
other executables are already fat with support for both x86_64 and
arm64.
First steps towards a clean install script
for Keyman for Mac. Some steps still out-
of-order for now, particularly opening
System Preferences to allow Keyman before
Keyman has run for first time.
