`dbpurge` fails due to missing actor in `dbauthz`

cc @Emyrk 

```
2023-04-05 16:09:02.621 [ERROR]	<./coderd/database/dbpurge/dbpurge.go:45>	New.func1	failed to purge old database entries ...
  "error": no authorization actor in context:
               github.com/coder/coder/coderd/database/dbauthz.init
                   /home/colin/Projects/coder/coder/coderd/database/dbauthz/dbauthz.go:22
             - sql: no rows in result set
```