Fix buggy globbing rules in addon build

[oskirby]

Description

I have noticed that the flatpak builds started failing after merging #10064 due to a failure to build the test addons, which I failed to notice because the flatpak builds were kinda flaky lately anyways. The problem seems to be that we have a buggy use of GLOB_RECURSE in addons.cmake that doesn't include any glob patterns and can fail to match any addons when CMAKE_CURRENT_SOURCE_DIR and ADDON_SOURCE_DIR differ.

While we're at it, we don't need to use inline python to parse the ADDON_ID. We have since upgraded to CMake > 2.20 which now includes native support for JSON parsing.

Reference

Example build failure: https://github.com/mozilla-mobile/mozilla-vpn-client/actions/runs/12019520242/job/33506273223

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

[mcleinman]

Thanks 👍🏻