Merge pull request #27374 from mnacamura/r_pbdZMQ_darwin_fix

r-pbdZMQ: fix Darwin install

pkgs/development/r-modules/default.nix

···
     pbdMPI = [ pkgs.openmpi ];
     pbdNCDF4 = [ pkgs.netcdf ];
     pbdPROF = [ pkgs.openmpi ];
+    pbdZMQ = [ pkgs.which ];
     PKI = [ pkgs.openssl.dev ];
     png = [ pkgs.libpng.dev ];
     PopGenome = [ pkgs.zlib.dev ];