[nix] Revert protoc-gen-grpc-web to 1.4.2 using overlay

Summary:
This creates an overlay with the last nixpkgs revision 9957cd48326fe8dbd52fdc50dd2502307f188b0d to have grpc-web version 1.4.2
and replaces the dev-shell installation which uses the latest nixpkgs-unstable grpc-web package.

Test Plan: Successfully ran nix develop and it installed 1.4.2 properly

Reviewers: varun, bartek

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10739