fix(#3422): stop pointing the bridge at a second user store

The bridge no longer has a private canonical store, so the env var
naming one is gone rather than repointed. What is left is the single
users file it now reads and writes directly.

That line is what made `swarm agent create` fail on this hive: the
bridge found no store at its own JSON path, saw users already in
`users.yml`, and refused to overwrite a file it had not written --
correctly, given two things claimed to be canonical for one file.

nix/host-modules/swarm-authelia.nix

@@ -920,14 +920,12 @@ in
             };
             environment = {
               SWARM_AUTHELIA_BRIDGE_BIND = "127.0.0.1:${toString cfg.bridgePort}";
-              # Own canonical store, alongside authelia's own state —
-              # NOT `swarm-controller`'s state dir: the two processes
-              # aren't guaranteed to be on the same host, and this store
-              # has to live wherever its writer (this bridge) does. See
-              # `swarm-authelia-bridge/README.md`'s "known limitation"
-              # section for the resulting `swarmctl`-owns-a-second-store
-              # seam.
-              SWARM_AUTHELIA_BRIDGE_STORE = "${stateDir}/swarm-authelia-bridge-users.json";
+              # ONE file, and this is it. The bridge used to carry a
+              # second, private `…-users.json` it treated as canonical
+              # while writing `users.yml` as a rendering of it — two
+              # canonical stores for one physical file, which is what
+              # made `swarm agent create` refuse to start on a hive whose
+              # `users.yml` already held users.
               SWARM_AUTHELIA_BRIDGE_USERS_FILE = cfg.usersFile;
               # The CONFIGURED authelia, not whatever is on `PATH`: the
               # argon2 parameters baked into a hash have to match the