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

Cargo.toml

@@ -657,6 +657,8 @@ features = [
 [patch.crates-io]
 cpal = { git = "https://github.com/zed-industries/cpal", rev = "fd8bc2fd39f1f5fdee5a0690656caff9a26d9d50" }
 real-async-tls = { git = "https://github.com/zed-industries/async-tls", rev = "1e759a4b5e370f87dc15e40756ac4f8815b61d9d", package = "async-tls" }
+notify = { git = "https://github.com/zed-industries/notify.git", rev = "bbb9ea5ae52b253e095737847e367c30653a2e96" }
+notify-types = { git = "https://github.com/zed-industries/notify.git", rev = "bbb9ea5ae52b253e095737847e367c30653a2e96" }
 
 [profile.dev]
 split-debuginfo = "unpacked"