commands: better env handling

[NotAShelf]

Safer, cleaner environment handling to fine-grain what is preserved. The goal is to allow specifying similar fine-graining for each command.

Fixes #314

[NotAShelf]

Okay I'm pretty positive that this works as intended now. CC @khaneliman for Darwin testing if possible, I've fixed Darwin tests but can't quite tell if environment handling is optimal for Darwin systems.

[hezhenxing]

Please also add NIXOS_INSTALL_BOOTLOADER to the default preserved list. And also, we need to make sure it is also preserved by sudo, so that we can set this variable to install the bootloader when switching configuration.

[NotAShelf]

Good catch, thank you @hezhenxing.

[kagurazakei]

ahh this pr was merged??
can i use the main branch again??

[NotAShelf]

No the PR is very much still open until I get a review.

[khaneliman]

Appears to be working for me on darwin

[NotAShelf]

Looks like viper is not reviewing this one, so merge on green it is.

[NotAShelf]

@khaneliman I'll try to review your PR over this week. I'm a little too busy with work, but I'll take a look the first chance I get.