build/bootstrap/.last-warned-change-id gets removed by workaround for #125578

[lolbinarycat]

bootstrap.py contains this snippet:

                # FIXME: A cheap workaround for https://github.com/rust-lang/rust/issues/125578,
                # remove this once the issue is closed.
                bootstrap_out = self.bootstrap_out()
                if os.path.exists(bootstrap_out):
                    shutil.rmtree(bootstrap_out)

                p.map(unpack_component, tarballs_download_info)

when updating to a new master, bootstrap gets rebuilt, causing the "last warned change id" file to be deleted, so you see all the "new changes to x.py", even if there are no new changes to x.py.