[Nix] Explicitly set emscripten cache

Summary:
Avoid emscripten trying to write to the nix store, which
will always fail

Depends on D8395

Test Plan:

cd web

yarn build-db-wasm

Reviewers: atul, jon

Reviewed By: jon

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8446