Noissue - Don't unpack browsers with download-test-browsers script

I noticed that we are currently caching each browser twice, as ensureFirefox() and ensureChromium() (called by npm run download-test-browsers which in turn is executed by the prepare-dependencies job) save the downloaded archive as well as the unpacked files. I have added a second optional argument to these functions that if set to false will cause the downloaded archives not to be unpacked.

This cuts down the duration of the prepare-dependencies job on the shared runners from ~13min to ~4min on a hot cache (or ~4.5min if browsers are downloaded).

For reference, it's not the unpacking itself that takes the bulk of the time, but to save a cache that includes the unpacked files for all browsers.