installation: Respect configured set of repo finders
installation: Respect configured set of repo finders

OSTree now has a repo config option to set which repo finders (that is,
sources for refs) will be used, and API which exposes the default set
(which was either configured by the user or decided by libostree). So
this commit changes flatpak_installation_list_remotes_by_type() to
respect that configuration, since that's the only place flatpak passes a
set of OstreeRepoFinder instances to libostree.

See also https://github.com/ostreedev/ostree/pull/1758/

Closes: #2264
Approved by: pwithnall

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