borgbackup: Add acl library to build inputs on linux platform

The acl libraray is only required by the borgbackup package if building on a
linux platform. Adding it only in this case should be fine. Also see the
conditional in the setup.py at [1].

[1] 72232a9bd5/setup.py (L768)

pkgs/tools/backup/borg/default.nix

@@ -16,8 +16,8 @@ python3Packages.buildPythonApplication rec {
     sphinx guzzle_sphinx_theme
   ];
   buildInputs = [
-    acl lz4 openssl python3Packages.setuptools_scm
-  ];
+    lz4 openssl python3Packages.setuptools_scm
+  ] ++ stdenv.lib.optionals stdenv.isLinux [ acl ];
   propagatedBuildInputs = with python3Packages; [
     cython llfuse msgpack
   ];