feat(deb,rpm) allow custom GPG public key file through a new environment variable 'GPG_PUBLIC_KEY_FILE' and fix missing Debian HTML

[dduportal]

Ref. jenkins-infra/helpdesk#4922

This PR ensures that the public GPG key used by the packaging process is:

• Published to the final webserver (pkg.jenkins.io) under the filename specified by the GPG_PUBLIC_KEY_FILE environment variable
  • It would have avoided the issue [pkg.origin.jenkins.io] puppet agent keeps updating the GPG jenkins-infra/helpdesk#3519 in 2023. And it will help us with the new 2026 key.
• Ensure the HTML pages for Debian and RPM are properly generated with the GPG key filename passed to the Python template engine
  • Includes a fix where the Debian HTML website was not uploaded to the correct location, so it was missing in our staging tests
  • Note: RPM HTML does not have a direct mention of the GPG key, but better to have it in case we change the documentation

Tested as part of jenkins-infra/release#829 with success:

• The new key is published under the name jenkins.io-2026.key
• The HTML content for Debian is present and mentions the new key filename (and its new ID and expiration date)

[lemeurherve]

🚀

[timja]

Not manually tested