Builds on Darwin have started to fail recently. This seems to be caused
by a check in Makefile that verifies whether the system is able to build
BoringSSL. This check attempts to compile test/build/boringssl.c, which
is absent in PyPI archives of grpcio.
This change builds the grpcio module directly from the grpc git
repository, so that all the files that are necessary for checks are
present.
Affects #56826 since darwin builds of this derivation on nixpkgs-19.03
fail.
grpcio currently does not build on Darwin (10.13.6). Due to the
following issues:
* ar is invoked with incorrect flags (#33765).
* libtool cannot be found, with a libtool dependency, with libtool
the option '-no_warning_for_no_symbols' is not recognized.
* the module build cannot find boringssl that is packaged with
python-grpcio when pkgconfig is not installed (grpc/#10058).
