internal: Cleanup cfg and env handling in project-model

[Veykril]

Fixes #16122 (comment)

miri and debug_assertions are now enabled via the cargo.cfgs config by default, allowing them to be disabled by overwriting the config.

[Veykril]

Additionally, this also enables the use of the cargo.cfgs for other workspace kinds (detached files and rust-project.json), though arguably, the name doesn't really make sense anymore now. 8989dcf

cc @davidbarsky for thoughts on that part.

[davidbarsky]

Additionally, this also enables the use of the cargo.cfgs for other workspace kinds (detached files and rust-project.json), though arguably, the name doesn't really make sense anymore now. 8989dcf

cc @davidbarsky for thoughts on that part.

Oh, I'm glad you made this change, but for naming, how does something like build.cfg feel?

(There's a few folks at work who wanted this; being able to point them to this setting would be immensely helpful!)

[Veykril]

build is probably the right prefix here, but I'll not add that for now (we'll need to keep supporting cargo.cfgs for some time after anyways)
@bors r+

[bors]

📌 Commit cdb8c3a has been approved by Veykril

It is now in the queue for this repository.

[bors]

⌛ Testing commit cdb8c3a with merge 50bdeaa...

[bors]

☀️ Test successful - checks-actions
Approved by: Veykril
Pushing 50bdeaa to master...