cache gets busted on treefmt-nix config changes

[zimbatm]

Describe the bug

The cache is stored in ~/.cache/treefmt/<hash-of-project.toml-path>.toml (see

treefmt/src/eval_cache.rs

Line 203 in e51789f

let treefmt_hash = Sha1::digest(path_bytes.as_bytes());

) With treefmt-nix, the config file gets a new path on every config change, invalidating the cache entirely.

A better solution would be to use the projectRoot path as a cache key.

[brianmcgee]

I don't believe this is an issue in v2 any more, but I need to double check.