Some cbuildbot builds are starting to use the workspace directory for a secondary checkout.

The workspace directory is correctly being created by cbuildbot_launch in the cbuild named cache as "cbuild/workspace" next to the standard "cbuild/repository".

If we instead move the builds that use workspaces to new LUCI Builders which list "workspace" as a named cache, and adjust our recipe to pass it along to cbuildbot_launch, then we can have workspace builds prefer builders with a hot workspace cache.

This has impact on performance, and will be more in the spirit of swarming builds, but will not affect build correctness.