feature: set environment variables as options

[15r10nk]

Is your feature request related to a problem? Please describe.

I use coverage.py and coverage-enable-subprocess to run my tests with coverage.
This requires a environment variable TOP to be set to the project root directory

Describe the solution you'd like

I would like to use a option like extra_env={...} to add environment variables

with ctx.options(extra_env={"TOP": str(Path.cwd())}):
    ctx.run(...) # TOP is used
# TOP is not used

I searched for something like this in your docs, because it is possible to set the cwd via an option and I expected that there would also be an option for environment variables.

Describe alternatives you've considered

My current solution is this

os.environ["TOP"]=str(Path.cwd())

This works, but I think the other solution would be cleaner.

Additional context

[pawamoy]

Sure, that's a good idea! I wonder if we should accept partial environments that are used to update the current one (temporarily), or complete envs that replace it, as the former would not allow removing environment variables. That's what subprocess does for example, and we usually do env={**os.environ.copy(), "MY_ENV": "VALUE"} to pass complete environments. A bit less ergonomic, for sure.