tools: use mailmap for find-inactive-collaborators

The current version of find-inactive-collaborators can generate a false
positive if the mailmap entry for a collaborator does not match the
entry in the README. (We should probably lint or otherwise check for
that sort of mismatch but regardless, it is relatively easy to avoid
having find-inactive-collaborators tripped up by it, so let's fix that
too, which is this commit.)

PR-URL: https://github.com/nodejs/node/pull/39432


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>