Fix test_run_with_command_flags on macOS (BSD printenv)

[ekanshul]

Fixes #708.

test_run_with_command_flags ran printenv --version to prove that dotenv run forwards flags instead of swallowing them. That only works with GNU printenv. On macOS, BSD printenv rejects --version (illegal option -- -) and the test fails even though forwarding is correct.

This switches the sentinel command to sys.executable --version, which behaves the same on Linux, macOS, and Windows.

[theskumar]

Thanks @ekanshul, the switch to sys.executable fixes the macOS failure. I checked it with a BSD-style printenv shim: main fails this test, and this branch passes the full suite.

While reviewing, I noticed the test couldn't catch the regression it was written for (#607). dotenv run has no --version option, so the pre-#607 code forwarded --version too. I've pushed a follow-up commit (e260e63) on top of yours that forwards --help (which run does define) and asserts the exact argv the command receives:

check_process(result, exit_code=0, stdout="['--help']\n")

Mutation results for test_run_with_command_flags (mutants applied to src/dotenv/cli.py):

Mutant	python --version	--help + argv (new)
pre-#607 run (only ignore_unknown_options)	missed	caught
allow_interspersed_args=True	missed	caught
forward only argv[0]	caught	caught

This also drops the stdout + stderr fallback: python --version writes to stdout on Python 3, and the new assertion is an exact stdout match.