Epic: Improve workspace starts with prebuilds on busy repos

[jldec]

Users are often blocked waiting for prebuilds when they try to start workspaces on repositories which see frequent commits and have a long prebuild time.

This epic aims to address the problem in stages.

• Allow workspaces to start from prebuilds built against older git commits #13900
• Epic: Incremental Workspaces #12582
• Run Fewer Prebuilds #12583

[jldec]

Related issues

• Avoid making users wait for long prebuilds #5941
• Avoid prebuilds for some commits #6815
• Feature Request: Skip prebuild loading #9789