Updates sometimes get stuck in pending state

[mattiaverga]

From quick look, it seems that the composer does:

1. move from f*-updates-candidate to f*-updates-testing
2. do stuff
3. untag from f*-updates-testing-pending
4. mark update state as testing

If the composer hang on 2 the update remains stuck in pending as the builds are not tagged anymore in f*-updates-candidate. We should find a solution.