siso: allow run $SISO_PATH even if .sisoenv not found

When $SISO_PATH is set, allow to run it even if .sisoenv not found.
Useful for `siso fetch`, `siso recall` etc, which would not be
needed to run in chromium checkout dir.

Change-Id: If3bfcf879cc67dc41bedf17cd771c94de836c949
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5804427
Commit-Queue: Richard Wang <richardwa@google.com>
Auto-Submit: Fumitoshi Ukai <ukai@google.com>
Commit-Queue: Fumitoshi Ukai <ukai@google.com>
Reviewed-by: Philipp Wollermann <philwo@google.com>
Reviewed-by: Richard Wang <richardwa@google.com>

siso.py

@@ -113,6 +113,8 @@ def main(args):
         for siso_path in siso_paths:
             if siso_path and os.path.isfile(siso_path):
                 return subprocess.call([siso_path] + args[1:], env=env)
+    if siso_override_path:
+        return subprocess.call([siso_override_path] + args[1:])
 
     print(
         'depot_tools/siso.py: Could not find .sisoenv under build/config/siso '