Create qt.conf file in $prefix/libexec on non-Windows platforms

With some tools being moved to libexec as per https://bugreports.qt.io/browse/QTBUG-88791 they need to be able to find the $prefix/bin folder.

The source of truth for the prefix and bin folder is the qt.conf file, which is created in $prefix/bin, and executables that are in $prefix/bin can successfully read it.

That's not the case for tools in $prefix/libexec.

The only sensible solution we found is to create a qt.conf file in ./libexec as well.

Given that the Installer creates the qt.conf file, we need it to be changed to create it also in the ./libexec dir in addition to the ./bin dir, on non-Windows platforms (because Windows doesn't have libexec).

I think this doesn't concern cross-compiling configurations like iOS or Android, so the target_qt.conf patching should not be relevant in this case.

Ideally we would create a hard link from ./bin/qt.conf to ./libexec/qt.conf, but a copy is probably fine too.

We should probably do the same for our Boot2Qt builds.