dolt fetch --prune doesn't work when sql-server running

[macneale4]

see: https://github.com/dolthub/dolt/blob/801a82a3af6e60d688e12238dec22bd5c9ea7e10/go/libraries/doltcore/env/actions/remotes.go#L742

For reasons which are not immediately clear the dolt fetch --prune command doesn't seem to actually prune when a sql-server is running. I think the race mentioned in the code above is the reason.