rrtmgp cmake is looking for netcdf

https://github.com/E3SM-Project/scream/blob/3005a4b8ef245c06f0a769a87c3c85bf0c29e519/components/eamxx/src/physics/rrtmgp/CMakeLists.txt#L127

the issue is that netcdf is already processed at the top level in cime, but then rrtmgp is trying to find it, too (also, not in the optimal way).

on aurora, for example, it leads to symbol `-lNETCDF_C-NOTFOUND` in link line because of
```
CMakeCache.txt:NETCDF_C:FILEPATH=NETCDF_C-NOTFOUND
```