autoninja: 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 autoninja.py

Change-Id: I5ce4c904aa11dd8172672ff9a8e9a06ebf69c9fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5599922
Reviewed-by: Erik Staab <estaab@chromium.org>
Commit-Queue: Fumitoshi Ukai <ukai@google.com>
Reviewed-by: Philipp Wollermann <philwo@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Reviewed-by: Gary Tong <gatong@chromium.org>
Reviewed-by: Gregory Guterman <guterman@google.com>

autoninja.py

@@ -207,6 +207,11 @@ def _gn_lines(output_dir, path):
 
 
 def main(args):
+    # 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")
     # The -t tools are incompatible with -j
     t_specified = False
     j_specified = False