nix: fix darwin devshell

.envrc

@@ -1,5 +1,5 @@
 # If we are a computer with nix-shell available, then use that to setup
 # the build environment with exactly what we need.
 if has nix; then
-  use nix
+  use flake
 fi

nix/devshell.nix

@@ -73,13 +73,14 @@ in mkShell rec {
     parallel
     python
     tracy
-    valgrind
     vttest
-    wraptest
 
     # wasm
     wabt
     wasmtime
+  ] ++ lib.optionals stdenv.isLinux [
+    valgrind
+    wraptest
   ];
 
   buildInputs = [