.ci/testcases/test_device.py: fix TypeError

Fix TypeError when RuntimeError about missing !archcheck is thrown

FAILED testcases/test_device.py::test_aports_device -
	TypeError: can only concatenate str (not "PosixPath") to str

.ci/testcases/test_device.py

@@ -61,7 +61,7 @@ def test_aports_device():
             raise RuntimeError("wrong architecture, please change to arch=\""
                                f"{deviceinfo.arch}\": {path}")
         if "!archcheck" not in apkbuild["options"]:
-            raise RuntimeError("!archcheck missing in options= line: " + path)
+            raise RuntimeError("!archcheck missing in options= line: " + path.as_posix())
 
 
 def test_aports_device_kernel():