fix rustfmt in darwin

Without this patch rustfmt installs successfully but upon running it
it throws the following error

  $ rustfmt
  dyld[68147]: Library not loaded: @rpath/librustc_driver-c577224f5690b349.dylib
    Referenced from: <no uuid> /nix/store/w46inc6cad6xg2kw09v9n629iqb7aqrw-rustfmt-1.69.0/bin/rustfmt
    Reason: tried: '/nix/store/ih18sy1kcbl5kdgq2l8l1lm70ai6aybj-apple-framework-CoreFoundation-11.0.0/Library/Frameworks/librustc_driver-c577224f5690b349.dylib' (no such file), '/System/Volumes/Preboot/Cryptexes/OS/nix/store/ih18sy1kcbl5kdgq2l8l1lm70ai6aybj-apple-framework-CoreFoundation-11.0.0/Library/Frameworks/librustc_driver-c577224f5690b349.dylib' (no such file), '/nix/store/ih18sy1kcbl5kdgq2l8l1lm70ai6aybj-apple-framework-CoreFoundation-11.0.0/Library/Frameworks/librustc_driver-c577224f5690b349.dylib' (no such file), '/System/Volumes/Preboot/Cryptexes/OS/nix/store/ih18sy1kcbl5kdgq2l8l1lm70ai6aybj-apple-framework-CoreFoundation-11.0.0/Library/Frameworks/librustc_driver-c577224f5690b349.dylib' (no such file), '/usr/local/lib/librustc_driver-c577224f5690b349.dylib' (no such file), '/usr/lib/librustc_driver-c577224f5690b349.dylib' (no such file, not in dyld cache)
  Abort trap: 6

Reading this [rust-issue] it seems that we have to link against
rustc_driver. The [clippy.nix] expression already does something similar

Of the 4 executables found in the result of building rustfmt only
rustfmt and git-rustfmt needed to be linked. The other worked without
linking to rustc_driver.

[rust-issue]: rust-lang/rust#105609
[clippy.nix]: https://github.com/NixOS/nixpkgs/blob/c8cf570dae9b41f30395b71f9b432418b4ff0ebc/pkgs/development/compilers/rust/clippy.nix#L27-L36

pkgs/development/compilers/rust/rustfmt.nix

@@ -15,6 +15,11 @@ rustPlatform.buildRustPackage rec {
     rustPlatform.rust.rustc.llvm
   ] ++ lib.optional stdenv.isDarwin Security;
 
+  preFixup = lib.optionalString stdenv.isDarwin ''
+    install_name_tool -add_rpath "${rustPlatform.rust.rustc}/lib" "$out/bin/rustfmt"
+    install_name_tool -add_rpath "${rustPlatform.rust.rustc}/lib" "$out/bin/git-rustfmt"
+  '';
+
   # As of 1.0.0 and rustc 1.30 rustfmt requires a nightly compiler
   RUSTC_BOOTSTRAP = 1;