Run Fewer Prebuilds

[svenefftinge]

(Idea from a discussion with @mbrevoort)

We currently run prebuilds on every push. This has two problems:

• highly frequented branches are constantly running prebuilds so they are constantly cancelled and there's rarely a usable prebuild.
• Many other prebuilds are also not used.

Proposed Solution

Run only the full prebuilds and allow workspaces to use them to start incremental workspaces. Make it so that incremental workspaces are based on the last successful prebuild, so if there's a running full prebuild on the commit it is not considered, i.e. I don't need to wait for it.

Benefits

• Fewer prebuilds
• No prebuilds in the hot path

[josepjaume]

Great idea! Would be a game changer for us!