Force nginx proxy_cache_use_stale to stay at default value.

Upstream float changes the default float proxy.conf in https://git.autistici.org/ai3/float/-/commit/615097155bac993f526394c6f1ea3b3e2ec5d305 that causes 404 errors reaching menshen. By simply adding the default value back to the menshen specific virtual host block, the problem is resolved, without impacting the rest of the float system.

This upstream commit is not yet in lilypad, but when it comes, this will be necessary. Having this in place before does nothing, as this is the default nginx value.

Fixes: leap/menshen#60 (closed)