Previously the builder scripts defined a readonly `VERSION` environment
variable for the Keyman version. That caused problems when another
(external) script tried to define a `VERSION` variable. We encountered
this problem when trying to move the TC build steps of a configuration
into a single script (#13399) when we tried to source `~/.nvm/nvm.sh`.
This change uses a Keyman specific prefix for the version variables and
renames `VERSION` → `KEYMAN_VERSION` etc. Unfortunately these variables
are used in a lot of places, so this turned out to be a bit of a yak
shave.
Test-bot: skip
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
```
