os: use AddCleanup, not SetFinalizer, for Process

os: use AddCleanup, not SetFinalizer, for Process

There is no reason to use a cleanup/finalizer for a Process that
doesn't use a handle, because Release doesn't change anything visible
about the process.

For #70907

Change-Id: I3b92809175523ceee2e07d601cc2a8e8b86321e0
Reviewed-on: https://go-review.googlesource.com/c/go/+/638579
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>