This way you can e.g. do network i/o in them.
Thats not a recommended way to do it though, as you work around
a lot of the feature in flatpak-builder like the caching and
verification.
This commit makes flatpak print a debug message (which only appears to
the user if the -v option is used) whenever a subprocess is launched.
This should make debugging easier, both for flatpak users and
developers.
