don't auto deduce QStringBuilder

see 'auto QString(Builder) considered VERY HARMFUL' thread on kde-devel
https://markmail.org/message/mbrp5ypyhcfk7zjn

What happens is that the type of str ends up being QStringBuilder
instead of QString. The QStringBuilder is destroyed after the semicolon,
and all access to "str" produces undefined behavior.

Never use auto for QString concatenation!

BUG: 439877