[cashtab] use npm ci instead of install on CI

Summary:
npm ci will clean node_modules directory and install dependencies from
package-lock.json file, ensuring builds are deterministic.

this should also be the workflow for building the app locally to
develop as to avoid spurious repo changes when the package.json file has
not been altered

Test Plan:

run on CI and check logs

Reviewers: #bitcoin_abc, alcipir, deadalnix, Fabien

Reviewed By: #bitcoin_abc, alcipir, deadalnix, Fabien

Subscribers: alcipir

Differential Revision: https://reviews.bitcoinabc.org/D8849