rust-bindgen: use rustfmt-nightly

bindgen's tests rely on specific output from rustfmt that uses
unstable features, so breaks when we build rustfmt as stable.

Also, use a better way to specify the rustfmt executable.

pkgs/development/tools/rust/bindgen/unwrapped.nix

@@ -1,4 +1,4 @@
-{ lib, fetchFromGitHub, rustPlatform, clang, rustfmt, writeTextFile
+{ lib, fetchFromGitHub, rustPlatform, clang, rustfmt-nightly
 , runtimeShell
 , bash
 }:
@@ -25,23 +25,11 @@ rustPlatform.buildRustPackage rec {
   '';
 
   doCheck = true;
-  checkInputs =
-    let fakeRustup = writeTextFile {
-      name = "fake-rustup";
-      executable = true;
-      destination = "/bin/rustup";
-      text = ''
-        #!${runtimeShell}
-        shift
-        shift
-        exec "$@"
-      '';
-    };
-  in [
-    rustfmt
-    fakeRustup # the test suite insists in calling `rustup run nightly rustfmt`
-    clang
-  ];
+  checkInputs = [ clang ];
+
+  # bindgen hardcodes rustfmt outputs that use nightly features
+  RUSTFMT = "${rustfmt-nightly}/bin/rustfmt";
+
   preCheck = ''
     # for the ci folder, notably
     patchShebangs .