Fix regex porting regression

QAbstractItemModel::match() under the hood uses QRegExp::exactMatch(),
so with QRegularExpression we need to use anchoredPattern() on the pattern.

Regression spotted by a failing dolphinmainwindowtest.
1 job for master in 4 minutes and 47 seconds (queued for 17 minutes and 13 seconds)
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passed linux kf5-qt5 qt5.15 #115643

00:04:47

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