build.plat: fix error in invoke_tool when running vivado.bat on Windows

amaranth/build/plat.py

@@ -338,7 +338,7 @@ def invoke_tool(context, name):
             if context.parent["syntax"] == "sh":
                 return f"\"${env_var}\""
             elif context.parent["syntax"] == "bat":
-                return f"\"%{env_var}%\""
+                return f"call %{env_var}%"
             else:
                 assert False