linux has a useful tool `env` to run a subprocess in a modified environment; i wrote a quick minimal version of it in python which might help windows users

env.py

#!/usr/bin/env python
import os
import sys
import argparse
import subprocess

desc = '''
run a program in a modified environment

example: run a tiny python script that reads the value of the variable "FOO"
  {prog} --vars FOO=42 -- python -c 'import os; print(os.environ[\"FOO\"])'
'''.format(prog=sys.argv[0]).strip()

def main(opts):
    if opts.verbose:
        print(opts)

    env = dict() if opts.ignore_environment else dict(os.environ)
    env.update(opts.vars)

    result = subprocess.run([opts.command] + opts.args, env=env)
    return result.returncode

def pair(s):
    name, _, value = s.partition('=')
    return name, value

def parse(argv):
    ap = argparse.ArgumentParser(
            formatter_class=argparse.RawTextHelpFormatter,
            description=desc
            )
    ap.add_argument('--vars', metavar='NAME=VALUE', type=pair, default=list(), nargs='*', help="Set each NAME to VALUE in the environment")
    ap.add_argument('--verbose', action='store_true')
    ap.add_argument('--ignore-environment', action='store_true')
    ap.add_argument('command', metavar='COMMAND', help="Program to run")
    ap.add_argument('args', metavar='ARG', nargs='*', help="Arguments to the program")
    return ap.parse_args(argv)

if __name__ == '__main__':
    sys.exit(main(parse(sys.argv[1:])))