nix: switch back to nightlies

shell.nix

@@ -1,13 +1,14 @@
 with import <nixpkgs> { overlays = [ (import <rust-overlay>) ]; };
 mkShell {
   buildInputs = [
-    (rust-bin.stable.latest.default.override {
-      extensions = [ "rust-src" "rustfmt-preview" ];
+    (rust-bin.selectLatestNightlyWith (toolchain:
+      toolchain.default.override {
+        extensions = [ "rust-src" "rustfmt-preview" "llvm-tools-preview"];
       targets =
         pkgs.lib.optionals pkgs.stdenv.isDarwin [ "aarch64-apple-darwin" ]
         ++ pkgs.lib.optionals pkgs.stdenv.isLinux
         [ "x86_64-unknown-linux-musl" ];
-    })
+    }))
     xorg.libxcb
     python39
   ];