fix(grfn/bbbg): Configure web base url

This was originally intended to work around the issue caused by me
accidentally ending up proxy_set_header'ing the Host header twice (which
nginx *concatenates with slashes*, rather than overwriting!), but seems
sensible regardless to make that whole thing (hopefully) a bit less
brittle

Change-Id: I877fa594b46e88d1ba05e793832beab3d0aaccdd
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4697
Reviewed-by: grfn <grfn@gws.fyi>
Autosubmit: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

users/grfn/bbbg/module.nix

@@ -71,6 +71,7 @@ in {
           PGUSER = cfg.database.user;
           PGDATABASE = cfg.database.name;
           PORT = toString cfg.port;
+          BASE_URL = "https://${cfg.domain}";
         };
 
         script = "${bbbg.server}/bin/bbbg-server";