community/grpc: fix CMAKE_INSTALL_PREFIX
Without this change /home/buildozer/aports/community/grpc/pkg/grpc/usr is used as a prefix in pkgconfig files, e.g. /usr/lib/pkgconfig/grpc++.pc which is obviously incorrect and causes application using this pkgconfig file to not compile.
This also requires setting DESTDIR in package() to make sure that the files are installed to the pkg/ directory.