bug: GetRunningPrebuiltWorkspaces creates lots of DB load

[spikecurtis]

In our dogfood deployment over 5 days in early June:

Avg Execution Time: 98.03ms
Num Times called: 205434
Avg Rows: 11

https://www.notion.so/coderhq/GetRunningPrebuiltWorkspaces-209d579be59280c5b98bc43e72874a48?source=copy_link