Revert "Revert "releasetools: Allow using futility-host to sign old TF.zip.""

This reverts commit d6145b6a.

The cause of the earlier breakage has been identified and fixed by
commit 4b577413.

Bug: 35467608
Test: lunch ryu-userdebug; m dist otatools-package
(cherry picked from commit 4f30f7cf)