Fix sandbox usage

Romainrequested to merge
rmnvgr/mat2:sandbox-fixes into master

Issues with Metadata Cleaner and mat2's sandbox in the latest GNOME Flatpak runtime lead me to investigate how mat2 is using its sandbox.

I found the following:

  • mat2's --no-sandbox argument is working in reverse (meaning by default no sandbox is used, and passing --no-sandbox would actually use a sandbox)
  • When a parser needs child parsers (in archives or FLAC), the sandbox option is not passed down

These commits should fix those issues.

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