fix sandbox test to avoid influence from other tests
add `make-environment-variables'

Swap order of argument for `environment-variables-get'
and `environment-variables-set!', so that the environment
variables come first --- which follows the usual order.
This change means that the parameter isn't used to get
the default environment variables, but that seems ok; the
convenient interface is `getenv' and `putenv'.

On Windows, case-normalized environment variable names.

Also, change the implementation to use an immutable hash
internally.