signers: Add missing MD5 digest to xpi metafiles
signers: Add missing MD5 digest to xpi metafiles

For backwards compat testing (Fx <58).

NB: AMO prod unaffected since this is only called from /sign/file with
one or more COSE algs

refs: https://github.com/mozilla-services/autograph/pull/121

mozilla-services/autograph,mozilla-services/autograph,mozilla-services/autograph