feat: add --clean-env flag to tasks and run command

[ruben-arts]

Fixes #289 slightly

Adds:

pixi run --clean-env xxx

[tasks]
isolated-task = {cmd = "/usr/bin/env", clean-env = true}

This doesn't work on Windows, as it is just to shitty to get working on windows.
You need to many variables and paths for a working environment for the label "clean" as you need the Program Files directories for the compilers. And other stuff.

And I'm honestly a 120% Done with this PR. And it's breaking into actually being productive.

[wolfv]

Maybe we should call it "clean-env", or "pristine" instead of isolated, so that we don't confuse the user in case we implement stronger isolation features (aka sandboxing)

[ruben-arts]

For some reason pristine sounds really funny to me. I'm fine with changing it. Some other options I could think of:

• minimal-env
• stripped-env
• empty-env
• all of the above without -env

[baszalmstra]

I think in terms of responsibility its a bit strange that the task graph needs to be aware of isolation. Cant we default it to false for the custom task?

[tdejager]

For some reason pristine sounds really funny to me. I'm fine with changing it. Some other options I could think of:

* `minimal-env`

* `stripped-env`

* `empty-env`

* all of the above without `-env`

clean-env could be another option? Otherwise I like stripped-env because it's explicit in what is done.

[wolfv]

I think I like clean-env or bare-env. It's pretty explicit about what it does.

[matthewfeickert]

Having this additional environment isolation is super nice so thank you very much for adding it into release v0.24.0+! ⭐