make it possible to fake envvars for testing

[actualben]

Currently we're always directly using os.environ, it would be useful for testing to enable using a different dict instead.

basecfg/src/basecfg/basecfg.py

Line 284 in 82fa138

if envvar_name in os.environ:

We do the same kind of thing with CLI args here:

basecfg/src/basecfg/basecfg.py

Line 82 in 82fa138

cli_args: Optional[Sequence[str]] = None,