Qt support: Don't set LD_LIBRARY_PATH for building

This does not make sense for non-Desktop versions, where it can cause
libraries from a sysroot to be erroneously picked up by executables in
build steps. Only set it for running desktop qt run configurations.

Change-Id: I3b098d5b1179eaefe20824b18a8976d68013d20d
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>