cmake: Add '-E env' command-line tool

Extend the cmake command-line interface to support

 cmake -E env [--unset=NAME]... [NAME=VALUE]... COMMAND [ARG]...

This will be useful to run processes with modified environments
without using a shell or a full "cmake -P" script to wrap it.

Extend the RunCMake.CommandLine test to cover success and failure cases.

Inspired-by: Jonathan Bohren <jbo@jhu.edu>
Merge topic 'cmake-E-env' into next

7abd5747 cmake: Add '-E env' command-line tool