When installing to an installation we need to find the runtime to use
for the apply-extra-data script from the installation we're targeting,
because that is the one that FlatpakTransaction guaranteed has the
required dependencies (although its possible they came from the
default dependency source too, i.e. the system repos).
In particular, we run into this issue if nothing is installed
anywhere, and then we install an extra-data app into a custom
directory. The transaction will download the runtime, and it
will not be anywhere else. Without this change flatpak only
looked for the dependency in the systam an regular user dirs
where it isn't.
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