[keyserver] await file operations for devs in MariaDB migrations

Summary:
We sometimes use MariaDB migrations to make updates to developers' environments.

The unused-promise Lint identified some cases where we don't await file operations. I think it makes sense to await these, so we can prevent the migration from continuing.

This could probably be optimized a bit, but I figure it doesn't really matter, since this code will probably never be executed again.

Depends on D10179

Test Plan: Flow

Reviewers: michal

Reviewed By: michal

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D10180