herwig: fix build by providing --with-boost (#441326)

Changed files
+1
pkgs
by-name
he
herwig
+1
pkgs/by-name/he/herwig/package.nix
···
configureFlags = [
"--with-thepeg=${thepeg}"
+
"--with-boost=${lib.getDev boost}"
];
enableParallelBuilding = true;