Add protobuf with corrected cmake build

Summary:
Unfortunately the protobuf 3.15 available in nix is using
an autotool build, which doesn't install the required protobuf-config.cmak
required by grpc to find the related package.

Upstreamed PR: https://github.com/NixOS/nixpkgs/pull/169497

Depends on D3793

Test Plan: nix build .#protobuf_3_15_cmake

Reviewers: varun, geekbrother, ashoat

Reviewed By: varun, ashoat

Subscribers: ashoat, palys-swm, Adrian, atul, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3865