stdenv: make darwin builds reproducable

Fixes #21629

Passing these extra linker flags removes both the semi-random uuid
included in most binaries as well as making the sdk version consistent
instead of based on the current os version.

    Load command 8
         cmd LC_UUID
     cmdsize 24
        uuid 70FAF921-5DC8-371C-B814-4F121FADFDF4

    Load command 9
          cmd LC_VERSION_MIN_MACOSX
      cmdsize 16
      version 10.12
          sdk 10.13

The -macosx_version_min flag isn't strictly necessary since that's
already handled by MACOSX_DEPLOYMENT_TARGET.