`apt update` fails because of misssing public key F6609E60DC62814E for rabbitmq-server

build-env

#1

Example build: https://travis-ci.org/yeputons/travis-bug-test/builds/465671192#L547

It simply runs apt update in before_install and gets the following error:

Err:105 https://packagecloud.io/rabbitmq/rabbitmq-server/ubuntu trusty InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY F6609E60DC62814E

It’s not especially painful here, because it only results in a warning: https://travis-ci.org/yeputons/travis-bug-test/builds/465671192#L557

W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://packagecloud.io/rabbitmq/rabbitmq-server/ubuntu trusty InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY F6609E60DC62814E

However, if I try to add gcc-7 by adding ubuntu-toolchain-r-test to addons.apt.sources, this warning becomes an error: https://travis-ci.org/yeputons/travis-bug-test/jobs/465671763#L617

E: The repository 'https://packagecloud.io/rabbitmq/rabbitmq-server/ubuntu trusty InRelease' is not signed.

Feel like Travis setup bug for me. Any suggestions, except for removing /etc/apt/sources.list.d/rabbitmq_rabbitmq-server.list like in this commit?