gh-105091: stable_abi.py: Remove "Unixy" check from --all on other platforms (GH-105092)

Author: encukou

Tools/build/stable_abi.py

@@ -684,7 +684,8 @@ def main():
 
     if args.all:
         run_all_generators = True
-        args.unixy_check = True
+        if UNIXY:
+            args.unixy_check = True
 
     try:
         file = args.file.open('rb')