Duplicate packages being build with different hashes because of different Autoconf provider

See https://github.com/spack/spack/issues/29930

I found this on the NOAA Cloud platforms. Pkgconf is defined as an external, but it's not being used in all instances, and `pkg-config` is being used, resulting in different hashes for NetCDF. My solution was to set `pkg-config` to `buildable: False`.