install: handle arch when installing from a flatpakref

Before this commit
$ flatpak install --arch someArch --from some.flatpakref
Would completely ignore the arch argument and only install for the
default flatpak architecture. Since flatpakref files don't include
an arch field, there's no reason we can't attempt to install the
arch specified on the command line.

(cherry picked from commit ee83b5a70d)
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