[ecash-lib] Add another stage to handle rust reqs for ecash-wasm build

Summary: Add another stage for wasm build

Test Plan: docker build -f ecash-lib.Dockerfile -t ecash-lib_local .

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16131