Avoid python stack traces when hitting ^C in autoupdate (#6004)

Currently hitting ^C prints out two stack traces, requiring scrolling up
though multiple screens of scrollback to get back to the autoupdate
results. This primarily shows up when hitting ^C while it's symbolizing
a C++ stack trace.

toolchain/autoupdate_testdata.py

@@ -96,4 +96,7 @@ def main() -> None:
 
 
 if __name__ == "__main__":
-    main()
+    try:
+        main()
+    except KeyboardInterrupt:
+        sys.exit(1)