Watch not working with admin user when using watchPg
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### Summary

I have confirmed that the connection string I'm using connects to a remote db as an admin user. When starting, Postgraphile gives me this warning:

```
Failed to setup watch fixtures in Postgres database ️️⚠️
This is likely because the PostgreSQL user in the connection string does not have sufficient privileges; you can solve this by passing the 'owner' connection string via '--owner-connection' / 'ownerConnectionString'. If the fixtures already exist, the watch functionality may still work.
Enable DEBUG='graphile-build-pg' to see the error
```

And here's the debug logs:


```
  graphile-build-pg error: permission denied to create event trigger "postgraphile_watch_ddl"
  graphile-build-pg     at Parser.parseErrorMessage (/Users/fang/Projects/mammal-core/node_modules/pg-protocol/src/parser.ts:369:69)
  graphile-build-pg     at Parser.handlePacket (/Users/fang/Projects/mammal-core/node_modules/pg-protocol/src/parser.ts:188:21)
  graphile-build-pg     at Parser.parse (/Users/fang/Projects/mammal-core/node_modules/pg-protocol/src/parser.ts:103:30)
  graphile-build-pg     at TLSSocket.<anonymous> (/Users/fang/Projects/mammal-core/node_modules/pg-protocol/src/index.ts:7:48)
  graphile-build-pg     at TLSSocket.emit (events.js:400:28)
  graphile-build-pg     at TLSSocket.emit (domain.js:470:12)
  graphile-build-pg     at addChunk (internal/streams/readable.js:290:12)
  graphile-build-pg     at readableAddChunk (internal/streams/readable.js:265:9)
  graphile-build-pg     at TLSSocket.Readable.push (internal/streams/readable.js:204:10)
  graphile-build-pg     at TLSWrap.onStreamRead (internal/stream_base_commons.js:188:23) +0ms
```

I'm using a Postgres 12 DigitalOcean managed database with a working admin connection string. Any reason why the permissions would be denied?


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### Additional context

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