feat: Add env to the tasks to specify tasks specific environment variables

[wolfv]

This adds the ability to set additional environment variables for a specific task. E.g.:

[tasks]
bla = { cmd = "echo $ABC", env = { ABC = "DEF" } }
path_mod = { cmd = "echo $PATH", env = { PATH="/tmp/path:$PATH" }}

This results in:

$ pixi run bla
DEF

$ ABC=hoi pixi run bla
hoi

$ pixi run path_mod
/tmp/path:/bin:/.local/bin .....

We might also want this for a feature / environment (as a lightweight way of adding an 'activation script').

TODO:

• Add tests, preferrably in unit and an example where it is tested on their test task.
• Add documentation
• Add to schema.json

[ruben-arts]

@wolfv Do you want to continue on this pr? other wise making an issue out of it and closing it would be nice.

[ruben-arts]

I like this, a lot actually.

A few questions:

• xxx = { cmd = "echo $ABC", env = { "ABC" = "asdsad" }} if i run this with ABC="hoi" pixi run xxx Should it say "hoi" or "asdsad"? I could imagine a nice use-case where it is used as a default but could be overwritten. This is also what task does: GREETING=hoi task greet uses hoi for GREETING instead of the value defined in taskfile.
• Should the env transfer in a depends_on graph? Taskfile doens't do that, so maybe for later

[wolfv]

LGTM, thanks for pushing. Also love the idea about using env vars as "positional arguments" and for task parameterization.

Just one note about the order of env vars.

[wolfv]

If I could, I would approve the PR (since I opened it I can't review!) :)