Dependencies are now cached between builds, using a checksum of the
`yarn.lock` file as the cache key. The `node_modules` directory and the
`.har` file from the install are cached and restored, so that we ensure
the record of the install is always preserved alongside the
dependencies.
The consolidation of the `collect-har-artifact` script was to make it
easier to cache the `.har` file along with the dependencies.
Mark Stacey
5e01602a01