Refactor: cts: Drop --set option from cts-lab

There's no clear use case for setting arbitrary "environment" variables.
These don't go into os.environ. They go into the global CTS Environment
instance, and they don't get used (for anything other than the dump()
function) unless they're also settable via other options.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>