Fall back to regular coreutils on Darwin

To prevent the following build error on MacOS x86_64
```
> error: don't yet have a `targetPackages.darwin.LibsystemCross for x86_64-apple-darwin`

ERROR: /Users/runner/work/rules_nixpkgs/rules_nixpkgs/WORKSPACE:45:16: fetching _nixpkgs_package rule //external:coreutils_static: Traceback (most recent call last):
  File "/private/var/tmp/_bazel_runner/a4a3182e50c9460fa52b8c6c1e6f916f/external/rules_nixpkgs_core/nixpkgs.bzl", line 589, column 31, in _nixpkgs_package_impl
    _nixpkgs_build_and_symlink(repository_ctx, [nix_build_path], expr_args, build_file_content)
  File "/private/var/tmp/_bazel_runner/a4a3182e50c9460fa52b8c6c1e6f916f/external/rules_nixpkgs_core/nixpkgs.bzl", line 447, column 34, in _nixpkgs_build_and_symlink
    exec_result = execute_or_fail(
  File "/private/var/tmp/_bazel_runner/a4a3182e50c9460fa52b8c6c1e6f916f/external/rules_nixpkgs_core/util.bzl", line 97, column 13, in execute_or_fail
    fail("""
Error in fail:
  Cannot build Nix derivation for package '@coreutils_static'.
    Command: "/nix/store/x5wfa3afc4l2rc7zx3wiiwcm5j3j4lbc-nix-2.17.0/bin/nix-build" "-I" "nixpkgs=/private/var/tmp/_bazel_runner/a4a3182e50c9460fa52b8c6c1e6f916f/external/nixpkgs/nixpkgs" "-E" "import <nixpkgs> { config = {}; overlays = []; }" "-A" "pkgsStatic.coreutils" "--out-link" "bazel-support/nix-out-link"
```
https://github.com/tweag/rules_nixpkgs/actions/runs/6390957120/job/17345262106?pr=423#step:6:846

Error message origin in nixpkgs:
https://github.com/aherrmann/nixpkgs/blob/59998678277cf465e749a144cf707021285bdf82/pkgs/top-level/all-packages.nix#L21799-L21802

WORKSPACE

@@ -37,7 +37,7 @@ nixpkgs_package(
 # `run-test-invalid-nixpkgs-package`.
 nixpkgs_package(
     name = "coreutils_static",
-    attribute_path = "pkgsStatic.coreutils",
+    nix_file_content = "let pkgs = import <nixpkgs> { config = {}; overlays = []; }; in if pkgs.stdenv.isDarwn then pkgs.coreutils else pkgsStatic.coreutils",
     repository = "@nixpkgs",
 )