Timeshift 2.011.1 which was released today, will not upgrade with Xapp 2.0.0. A work-around is to downgrade Xapp to 1.8.10 and then build Timeshift. After the upgrade, then you can upgrade Xapp back to 2.0.0. 
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To follow up, the PKGBUILD has now been updated and will build without manual intervention:
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