Without this change, it is non-trivial to know during
runhooks, if a landmine was just triggered in a checkout
that doesn't have the initial landmines script CL yet, i.e.
that didn't create a .landmines file yet.

BUG=chromium:403263
LOG=n

Review URL: https://codereview.chromium.org/954153002

Cr-Commit-Position: refs/heads/master@{#26842}