Upload keys to keys.opengpg.org for better disoverability

[pietrushnic]

To make keys discoverable, we should, with every PR in CI, upload those to the public keyserver. It may simplify signature verification for some users and employees' key discoverability, e.g. Thunderbird users.

https://keys.openpgp.org/about/usage#gnupg

[pietrushnic]

@macpijan do you think we can address this? @krystian-hebel show API which could be used in linked comment.

[tlaurion]

ping @macpijan, see linuxboot/heads#1804