pkg/api/grpcpb: auto-format generated protobuf files

Update build.sh to run gofumpt on generated .pb.go files, and apply
formatting to existing generated files. This fixes gofumpt and inamedparam
lint errors on macOS CI and ensures future regenerations will be
automatically formatted.

Signed-off-by: Lokesh Mandvekar <lsm5@linux.com>