sim: nrun: add --env-{set,unset,clear} command line options

Provide explicit control over the program's environment with the
basic set/unset/clear options.  These are a bit clunky to use,
but they're functional.

The env set operation is split out into a separate function as it'll
be used in the next commit.

With these in place, we can adjust the custom cris testsuite to use
the now standard options and not its one-off hack.