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