Add env attribute to set environment variables for recipes

[neunenak]

The env attribute accepts two arguments (env_var, value) and can be used multiple times per recipe to set environment variables without using the existing $parameter syntax:

[env('API_KEY', 'secret')]
[env('LOG_LEVEL', 'debug')]
deploy:
    ./deploy.sh

Addresses #2825

[neunenak]

Also as a general note, I initially generated this PR with LLM coding tools and then manually audited+tweaked some things. I dunno if just as a project does or should have some kind of stated policy on that, but I wanted to mention that for the sake of transparency.

I'm personally pretty bullish on AI-assisted coding, I do it all the time myself, and I take the position that in general a human developer is responsible for the code they submit to a project, but the project shouldn't care exactly how they generated that code.