Absolute paths for -kb argument should error consistently (#23262)

lib/python/qmk/path.py

@@ -12,7 +12,15 @@ from qmk.errors import NoSuchKeyboardError
 def is_keyboard(keyboard_name):
     """Returns True if `keyboard_name` is a keyboard we can compile.
     """
-    if keyboard_name:
+    if not keyboard_name:
+        return False
+
+    # keyboard_name values of 'c:/something' or '/something' trigger append issues
+    # due to "If the argument is an absolute path, the previous path is ignored"
+    # however it should always be a folder located under qmk_firmware/keyboards
+    if Path(keyboard_name).is_absolute():
+        return False
+
     keyboard_path = QMK_FIRMWARE / 'keyboards' / keyboard_name
     rules_mk = keyboard_path / 'rules.mk'