meld:master commits

Use appstream for appdata validation test See merge request !135

Update macOS job for new CI runner See merge request !136

Reconfigure cache settings to accomodate the new ephemeral infrastructure. Remove rules to restrict CI to specific namespaces. Use the default WRK_DIR (by removing that setting) which is /Users/Shared/work. This wasn't possible before on the shared machine. It enables us to use a pre-built version of an already bootstrapped JHBuild, speeding up build times. No more need to force building everything from source.