python3Packages.matplotlib: switch to use an absolute path in MPLSETUPCFG

The upstream made a transition setup.cfg -> mplsetup.cfg
https://github.com/matplotlib/matplotlib/commit/93b5c0fb08b5426623fa3927a515eaec0aac0c36

This should allow us to be compatible with some reasonable version overrides
https://github.com/NixOS/nixpkgs/pull/147742#issuecomment-995178116

pkgs/development/python-modules/matplotlib/default.nix

@@ -87,11 +87,7 @@ buildPythonPackage rec {
     };
   };
 
-  mplsetup_cfg = writeText "mplsetup.cfg" (lib.generators.toINI {} passthru.config);
-
-  preBuild = ''
-    cp "$mplsetup_cfg" ./mplsetup.cfg
-  '';
+  MPLSETUPCFG = writeText "mplsetup.cfg" (lib.generators.toINI {} passthru.config);
 
   # Matplotlib tries to find Tcl/Tk by opening a Tk window and asking the
   # corresponding interpreter object for its library paths. This fails if