build not idempotent: #speakers room alias already taken

[reivilibre]

There was an error processing your command: Error whilst creating #speakers:fosdem.org: {"errcode":"M_ROOM_IN_USE","error":"Room alias already taken"}. Tried to create the room because failed to resolve it: {"errcode":"M_UNKNOWN","error":"Failed to fetch alias"} when using build command.

Should ensure this is actually idempotent — I must have missed these special rooms in the last crack at doing this.

[reivilibre]

This is actually a non-fatal error, it will continue with the building.
It's already trying to be idempotent, but it's been configured to look for real fosdem.org as the serverpart of the alias (even though the server is the staging server), so it tries anyway.