Allow querying for alternate Qt6 libexec directory

This is closer to how Qt6's moc.prf finds moc, i.e.

```
qtPrepareLibExecTool(QMAKE_MOC, moc)

...

defineTest(qtPrepareLibExecTool) {
    isEmpty(instloc): instloc = "$$[QT_HOST_LIBEXECS]"
    qtPrepareTool($$1, $$2, $$3, $$4, $$instloc)
}
```

scripts/src/osd/modules.lua

@@ -384,12 +384,16 @@ function qtdebuggerbuild()
 			if _OPTIONS["QT_HOME"]~=nil then
 				MOCTST = backtick(_OPTIONS["QT_HOME"] .. "/bin/moc --version 2>/dev/null")
 				if (MOCTST=='') then
-					MOCTST = backtick(_OPTIONS["QT_HOME"] .. "/libexec/moc --version 2>/dev/null")
+					local qt_host_libexecs = backtick(_OPTIONS["QT_HOME"] .. "/bin/qmake -query QT_HOST_LIBEXECS")
+					if (qt_host_libexecs=='') then
+						qt_host_libexecs = _OPTIONS["QT_HOME"] .. "/libexec"
+					end
+					MOCTST = backtick(qt_host_libexecs .. "/moc --version 2>/dev/null")
 					if (MOCTST=='') then
 						print("Qt's Meta Object Compiler (moc) wasn't found!")
 						os.exit(1)
 					else
-						MOC = _OPTIONS["QT_HOME"] .. "/libexec/moc"
+						MOC = qt_host_libexecs .. "/moc"
 					end
 				else
 					MOC = _OPTIONS["QT_HOME"] .. "/bin/moc"