Fix a python invocation that wasn't replaced with vpython.

This fixes autoninja on my Windows machine at home.

Bug: none
Change-Id: I3c245715c409bb2b30fcc3733bb097fc2a88f464
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2021755
Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>

autoninja

@@ -5,7 +5,7 @@
 # found in the LICENSE file.
 
 # Set unique build ID.
-AUTONINJA_BUILD_ID="$(python -c "import uuid; print(uuid.uuid4())")"
+AUTONINJA_BUILD_ID="$(vpython -c "import uuid; print(uuid.uuid4())")"
 export AUTONINJA_BUILD_ID
 
 if [ "$NINJA_SUMMARIZE_BUILD" == "1" ]; then