Revert "CI: disable Flatpak build"
Revert "CI: disable Flatpak build"

This reverts commit 978e8c143185a93fee0ce51378f76fda0e06aae0. I think
bb27784f454cc348aad67bcccaca5c323c85f561 will fix this problem.

wjt/bustle,wjt/bustle