siso: set PYTHONDONTWRITEBYTECODE=1 if not set

during build, lots of python scripts run and generates
lots of pycache *.pyc in workspace.
To keep workspace clean, set PYTHONDONTWRITEBYTECODE=1
in siso.py

Change-Id: I725c6745c44410eef4a780b8ffc30fdafd899334
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5606824
Reviewed-by: Philipp Wollermann <philwo@google.com>
Reviewed-by: Erik Staab <estaab@chromium.org>
Commit-Queue: Fumitoshi Ukai <ukai@google.com>
Auto-Submit: Fumitoshi Ukai <ukai@google.com>

siso.py

@@ -63,6 +63,12 @@ def main(args):
         os.environ.pop("LIBRARY_PATH", None)
         os.environ.pop("SDKROOT", None)
 
+    # if user doesn't set PYTHONPYCACHEPREFIX and PYTHONDONTWRITEBYTECODE
+    # set PYTHONDONTWRITEBYTECODE=1 not to create many *.pyc in workspace
+    # and keep workspace clean.
+    if not os.environ.get("PYTHONPYCACHEPREFIX"):
+        os.environ.setdefault("PYTHONDONTWRITEBYTECODE", "1")
+
     environ = os.environ.copy()
 
     # Get gclient root + src.