pythonPackages.magic: Fix loading libmagic.so

Even with the previous `python.patch` file around, libmagic.so isn't
found. Instead patch it to use an absolute path, using a similar
approach that is used by the `python_magic` package (which is another
libmagic python wrapper library as well).

pkgs/tools/misc/file/python.patch

@@ -1,11 +0,0 @@
---- file-5.11/python/magic.py	2011-09-22 18:57:41.000000000 +0200
-+++ file-5.11/python/magic.py.1	2012-04-30 15:00:31.750213810 +0200
-@@ -13,7 +13,7 @@
-     Loads the shared library through ctypes and returns a library
-     L{ctypes.CDLL} instance 
-     """
--    return ctypes.cdll.LoadLibrary(find_library('magic'))
-+    return ctypes.CDLL('libmagic.so')
- 
- _libraries = {}
- _libraries['magic'] = _init()

pkgs/top-level/python-packages.nix

@@ -7042,7 +7042,10 @@ let
 
     src = pkgs.file.src;
 
-    patches = [ ../tools/misc/file/python.patch ];
+    patchPhase = ''
+      substituteInPlace python/magic.py --replace "find_library('magic')" "'${pkgs.file}/lib/libmagic.so'"
+    '';
+
     buildInputs = with self; [ python pkgs.file ];
 
     preConfigure = "cd python";