`buildGoModule` and `buildGoPackage` by default inherit the `platforms`
from go. That seems better than explicitly configuring `platforms.all`.
There are also many packages that specify 'linux + darwin' - this is
even suggested in the documentation. We might also want to update those,
but let's do the noncontroversial change first.
Using buildGoModule did not work for this package, and produced the
error "main module (google.golang.org/grpc) does not contain package
google.golang.org/grpc/cmd/protoc-gen-go-grpc". Since buildGoModule and
buildGoPackage both seem in common use and both have their advantages
(https://github.com/NixOS/nixpkgs/issues/84826) just using
buildGoPackage seems like a neat solution here.
