also try to find kioslave5 via ld_library_path

on our new gitlab ci dependee project's tests can't necessarily find libexec through the established means because the ultimate path where KIO will be unpacked to is different from both the build dir and the system dir.

e.g. $sources/_build/bin/testbinary wants to use KIO but the actual KIO libexec is in $sources/_install/lib/libexec/kf5/

LD_LIBRARY_PATH is set for the builds anyway, so we may as well try to derive libexec from it