fix(busbar-150): supply BUSBAR_ADMIN_TOKEN - 1.5.0's default admin_auth demands it

The box failed to boot twice with:

  [error] auth.admin_auth admin-tokens token did not resolve: secret
  env:BUSBAR_ADMIN_TOKEN cannot resolve: environment variable is unset

The rendered config on the box carries only 'auth: {chain: []}' - I read it off
the machine to be sure - so the admin_auth block is 1.5.0's own default, and an
unresolvable secret reference in it is fatal even when the data-plane chain is
empty and the benchmark never touches the admin API.

Supplying the variable is the minimum that satisfies it. It changes nothing on
the measured path: the admin surface is a separate listener the load generator
never calls, and the data-plane chain stays empty to match what 1.4.1 ran.

Author: MattJackson

gateways/busbar-150/definition.json

@@ -40,6 +40,10 @@
       [
         "BUSBAR_PROVIDERS",
         "/work/providers.yaml"
+      ],
+      [
+        "BUSBAR_ADMIN_TOKEN",
+        "bench-admin-token"
       ]
     ],
     "image": "getbusbar/busbar:1.5.0",