Add staging keyring package too

[oskirby]

Description

This adds a second output to the keyring package that bundles the GPG keys used to access the Mozilla staging package
repository, where we should now able able to fetch the release candidates of the static linux builds.

I am not 100% sure this is really needed given the temporary nature of this repository, and we hope to eventually use different package names (eg: mozillavpn-beta vs mozillavpn) in the prod repo for this purpose.

Reference

JIRA issue VPN-6365

Checklist

• My code follows the style guidelines for this project
• I have not added any packages that contain high risk or unknown licenses (GPL, LGPL, MPL, etc. consult with DevOps if in question)
• I have performed a self review of my own code
• I have commented my code PARTICULARLY in hard to understand areas
• I have added thorough tests where needed