perlmutter: remove explicit setting of cc and CC for Sowing

This MR removes lines from the Configure example for Perlmutter at NERSC that set cc and CC explicitly for Sowing; these are no longer needed (and I also had a typo in them, anyway -- d'oh!).

It also adds to the example some lines specifying that the develop branch for Kokkos and Kokkos Kernels be used; the current release branch will not build on Perlmutter.