gitian: make linux qt intermediate deterministic

A qt installation date snuck into the host utils (lrelease etc). This doesn't affect the end product, so no dependency version bump.

It also doesn't explain why gavin's and mine bitcoin-qt build is different:
https://github.com/bitcoin/gitian.sigs/blob/master/0.9.2rc1/laanwj/bitcoin-build.assert
https://github.com/bitcoin/gitian.sigs/blob/master/0.9.2rc1/gavinandresen/bitcoin-build.assert

Let's wait for some further gitian builds to see if they converge into two groups, or everyone gets wildly different output.