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

nix/shell.nix

@@ -1,7 +1,5 @@
 {
   mkShell,
-  stdenv,
-  stdenvAdapters,
   makeFontsConf,
 
   zed-editor,
@@ -12,12 +10,7 @@
   protobuf,
   nodejs_22,
 }:
-let
-  moldStdenv = stdenvAdapters.useMoldLinker stdenv;
-  mkShell' =
-    if stdenv.hostPlatform.isLinux then mkShell.override { stdenv = moldStdenv; } else mkShell;
-in
-mkShell' {
+(mkShell.override { inherit (zed-editor) stdenv; }) {
   inputsFrom = [ zed-editor ];
   packages = [
     rust-analyzer