[nix] Enable C++17 support for Emscripten

Summary:
Emscripten is coupled with clang version, according to clang docs C++17 starts from Clang 5.0, while we are using Clang 14.0.6 so should be safe.

Depends on D8487

Test Plan: Run script

Reviewers: michal, tomek, jon

Reviewed By: michal

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D8488