Disabled PythonProbe when not compiling for Windows

The message "WARNING: PYTHONHOME not set. The Python plugin will not be
available." is confusing, since on Linux the probe always fails, but the
Python plugin will generally be available given that it will be detected
through PkgConfigProbe instead.

qbs/imports/PythonProbe.qbs

@@ -6,6 +6,9 @@ import qbs.Utilities
 
 Probe {
     id: pythonDllProbe
+
+    condition: qbs.targetOS.contains("windows")
+
     property string pythonDir: pythonInstallDir // Input
     property string buildVariant: qbs.buildVariant // Input
     property string fileNamePrefix // Output