fix: Use stdenv.isLinux for platform check

Use stdenv.isLinux instead of lib.isLinux to properly check if
inotify-tools should be included. This fixes the undefined variable
error when evaluating the flake on macOS.

nix/shell.nix

@@ -1,5 +1,6 @@
 {
   lib,
+  stdenv,
   mkShell,
   writeShellScriptBin,
   # Elixir/Erlang
@@ -15,6 +16,7 @@
   pkg-config,
   # Development tools
   git-lfs,
+  inotify-tools,
   # LSPs and formatters
   elixir-ls,
   nixfmt,
@@ -221,7 +223,7 @@ mkShell {
     # Service management scripts
     startServices
     stopServices
-  ] ++ lib.optionals lib.isLinux [
+  ] ++ lib.optionals stdenv.isLinux [
     # Linux-only tools
     inotify-tools # For Mix file watching
   ];