cygwin.mirror.constant.com seems broken

[dmbaturin]

Uses of the CI script are currently failing with the following error:

mbox note: Mirror Error:  Setup.ini signature https://cygwin.mirror.constant.com/x86_64/setup.xz.sig from https://cygwin.mirror.constant.com/ failed to verify.
Possible corrupt mirror?  Setup.ini rejected.

For a live example: https://ci.appveyor.com/project/gildor478/ocaml-gettext/builds/47405296/job/rs1dyf5dilk4n6hn