chore: fix concurrent CommitQuota transactions for unrelated users/orgs (#15261)

The failure condition being fixed is `w1` and `w2` could belong
to different users, organizations, and templates and still cause a
serializable failure if run concurrently. This is because the old query 
did a `seq scan` on the `workspace_builds` table. Since that is the 
table being updated, we really want to prevent that.

So before this would fail for any 2 workspaces. Now it only fails if
`w1` and `w2` are owned by the same user and organization.

coderd/database/dbmem/dbmem.go

@@ -339,6 +339,10 @@ func (*FakeQuerier) Ping(_ context.Context) (time.Duration, error) {
 	return 0, nil
 }
 
+func (*FakeQuerier) PGLocks(_ context.Context) (database.PGLocks, error) {
+	return []database.PGLock{}, nil
+}
+
 func (tx *fakeTx) AcquireLock(_ context.Context, id int64) error {
 	if _, ok := tx.FakeQuerier.locks[id]; ok {
 		return xerrors.Errorf("cannot acquire lock %d: already held", id)