x/build: auto-detect flakes

[bradfitz]

Request from @danp:

When the build system is idle, run builds of tip branches periodically (every 5 min?) to see if builds alternate between "ok" and "failed" state. Maybe auto-file flake bugs if test failures aren't consistent.

[bradfitz]

TryBots should also auto-detect flakes and re-run anything that fails to see if it fails in the same way twice.

/cc @jessfraz @adams-sarah @cybrcodr @andybons

[dmitshur]

I think we can call this done. https://go.dev/wiki/Watchflakes