Broken signatures on packages.sury.org

[Matthew Pounsett]

Beginning today, I'm seeing the following errors on systems that use
the ISC Debian packages:

Err:5 https://packages.sury.org/bind buster InRelease
  The following signatures were invalid: EXPKEYSIG B188E2B695BD4743
DEB.SURY.ORG Automatic Signing Key <deb at sury.org>

I haven't seen any official word from ISC that there are new keys to
grab, so wanted to check that someone isn't messing about with your
repository.