beetsPackages.copyartifacts: Put beets in nativeBuildInputs

Has no real difference on the results, but it makes a bit more sense to
not remove beets from the setup.py file, and depend on it only via
nativeBuildInputs. This is also consistent with other plugins, see:

https://github.com/NixOS/nixpkgs/pull/257943#discussion_r2083635799

pkgs/tools/audio/beets/plugins/copyartifacts.nix

@@ -19,7 +19,6 @@ python3Packages.buildPythonApplication rec {
   };
 
   postPatch = ''
-    sed -i -e '/install_requires/,/\]/{/beets/d}' setup.py
     sed -i -e '/namespace_packages/d' setup.py
     printf 'from pkgutil import extend_path\n__path__ = extend_path(__path__, __name__)\n' >beetsplug/__init__.py
 
@@ -28,13 +27,15 @@ python3Packages.buildPythonApplication rec {
     sed -i -e 's/util\.py3_path/os.fsdecode/g' tests/_common.py
   '';
 
+  nativeBuildInputs = [
+    beets
+  ];
   build-system = with python3Packages; [ setuptools ];
 
   dependencies = with python3Packages; [ six ];
 
   nativeCheckInputs = [
     python3Packages.pytestCheckHook
-    beets
     writableTmpDirAsHomeHook
   ];