Ref T11613. In D16503/T11598 I refined the setup flow to improve messaging for early-stage setup issues, but failed to fully untangle things.
We sometimes still try to access a cache which uses configuration before we build configuration, which causes an error.
Instead, store "are we in flight / has setup ever worked?" in a separate cache which doesn't use the cache namespace. This stops us from trying to read config before building config.
