Set preventAssignment in @rollup/plugin-replace

[jeffposnick]

R: @tropicadri @philipwalton

Fixes #2769

The latest minor release of @rollup/plugin-replace has started logging messages about the new preventAssignment option, apparently in advance of some breaking changes in the next major release.

Folks using Workbox who end up with @rollup/plugin-replace v.2.4.0+ installed will get this message.

This PR explicitly updates our workbox-build and repo-level dependencies to require @rollup/plugin-replace v2.4.1, and explicitly sets preventAssignment: true, which should stop the logged warning messages.

(Based on how we're using @rollup/plugin-replace, preventAssignment: true should not result in any noticeable change.)