siso: use stderr to print message from siso.py

siso binary will print json etc (e.g. `siso fs export`)
and usually used to pipe to jq or so.

but siso.py print message when $SISO_PATH is set
and corrupt json message.

use stderr for such message.

Change-Id: Ic35ecabc159fb2053a8a29816c276d914f8d2468
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5197566
Auto-Submit: Fumitoshi Ukai <ukai@google.com>
Commit-Queue: Fumitoshi Ukai <ukai@google.com>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>

siso.py

@@ -52,7 +52,8 @@ def main(args):
     siso_override_path = os.environ.get('SISO_PATH')
     if siso_override_path:
         print('depot_tools/siso.py: Using Siso binary from SISO_PATH: %s.' %
-              siso_override_path)
+              siso_override_path,
+              file=sys.stderr)
         if not os.path.isfile(siso_override_path):
             print(
                 'depot_tools/siso.py: Could not find Siso at provided '