R1:96a8b89fa8a8

When building a config stack, stop SiteSource objects from poisoning the cache

Summary:
Ref T13168. I'm not sure how this worked before, but I ran into this issue on my new laptop.

SiteSource accesses `PhabrictatorEnv::getEnvConfig('phabricator.base-uri')` when local, which may poison the cache and lock the value since we don't later discard the cache.

Specifically, when I access `http://locala.phacility.com`, I was getting an error like "You made a request for locala.phacility.com, but no configured…