pick-ui: move NominationType values around so that None is 0

This keeps tripping me up when reading the `.pick_status.json`, so let's
change it around so that starting in the next release cycle, `NONE = 0`.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30427>

bin/pick/core.py

@@ -71,10 +71,10 @@ class PickUIException(Exception):
 @enum.unique
 class NominationType(enum.Enum):
 
-    CC = 0
-    FIXES = 1
-    REVERT = 2
-    NONE = 3
+    NONE = 0
+    CC = 1
+    FIXES = 2
+    REVERT = 3
     BACKPORT = 4