nix: Clean up build (#27881)

- bump our livekit version to include a fix for a crane bug (TODO: add
link when an issue is filed on crane)
- switch to a clang stdenv for both linux and macos
- manually unify versions of our notify crate
- remove old linker flags which were only needed for livekit
- fix an issue where RUSTFLAGS shadowed the rustflags from cargo configs

Release Notes:

- N/A

crates/fs/Cargo.toml

@@ -40,7 +40,7 @@ objc = "0.2"
 cocoa = "0.26"
 
 [target.'cfg(not(target_os = "macos"))'.dependencies]
-notify = { git = "https://github.com/zed-industries/notify.git", rev = "bbb9ea5ae52b253e095737847e367c30653a2e96" }
+notify = "8.0.0"
 
 [target.'cfg(target_os = "windows")'.dependencies]
 windows.workspace = true