Checkout fast was slower and checkout-and-save-cache sometimes didn't save a cache... Basically this unifies our checkout-and-save-cache steps under the existing electron-build command so less steps are duplicated and renames checkout-fast and checkout-and-save-cache to be more aligned with what they actually do.

* `checkout-fast` --> `checkout-for-workspace`: Checks out a fully synced directory and then persists to a workspace layer
* `checkout-and-save-cache` --> `make-src-cache`: Ensures that there is a valid and accurate src cache for this commit.  It doesn't always check it out, sometimes it early outs via the src-cache-marker

Notes: no-notes