pinocchio: set MPLCONFIGDIR in check

fix:

> Matplotlib created a temporary cache directory at
> /private/tmp/nix-build-pinocchio-3.2.0.drv-0/matplotlib-cgeg512t
> because the default path (/homeless-shelter/.matplotlib) is not a
> writable directory; it is highly recommended to set the MPLCONFIGDIR
> environment variable to a writable directory, in particular to speed
> up the import of Matplotlib and to better support multiprocessing.

pkgs/development/libraries/pinocchio/default.nix

@@ -41,6 +41,9 @@ stdenv.mkDerivation (finalAttrs: {
 
     # allow package:// uri use in examples
     export ROS_PACKAGE_PATH=${example-robot-data}/share
+
+    # silence matplotlib warning
+    export MPLCONFIGDIR=$(mktemp -d)
   '';
 
   # CMAKE_BUILD_TYPE defaults to Release in this package,