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/livekit_client/.cargo/config.toml

@@ -1,2 +0,0 @@
-[livekit_client_test]
-rustflags = ["-C", "link-args=-ObjC"]

crates/livekit_client/Cargo.toml

@@ -39,8 +39,8 @@ tokio-tungstenite.workspace = true
 util.workspace = true
 
 [target.'cfg(not(all(target_os = "windows", target_env = "gnu")))'.dependencies]
-libwebrtc = { rev = "102ebbb1ccfbdbcb7332d86dc30b1b1c8c01e4f8", git = "https://github.com/zed-industries/livekit-rust-sdks" }
-livekit = { rev = "102ebbb1ccfbdbcb7332d86dc30b1b1c8c01e4f8", git = "https://github.com/zed-industries/livekit-rust-sdks", features = ["__rustls-tls"] }
+libwebrtc = { rev = "ffc215666621cd903a689d1c56cce0a706ffe022", git = "https://github.com/zed-industries/livekit-rust-sdks" }
+livekit = { rev = "ffc215666621cd903a689d1c56cce0a706ffe022", git = "https://github.com/zed-industries/livekit-rust-sdks", features = ["__rustls-tls"] }
 
 [target.'cfg(target_os = "macos")'.dependencies]
 core-foundation.workspace = true