Use depot_tools python3 to call ninja.py

This guarantees the python version used to call ninja.py.

Bug: 1398687
Change-Id: Ife58b285bb621dddb20b947010bc19818cd41395
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4091206
Commit-Queue: Junji Watanabe <jwata@google.com>
Reviewed-by: Nico Weber <thakis@chromium.org>
Auto-Submit: Junji Watanabe <jwata@google.com>
Commit-Queue: Nico Weber <thakis@chromium.org>

ninja

@@ -5,4 +5,4 @@
 # found in the LICENSE file.
 
 base_dir=$(dirname "$0")
-PYTHONDONTWRITEBYTECODE=1 exec python3 "$base_dir/ninja.py" "$@"
+PYTHONDONTWRITEBYTECODE=1 exec "$base_dir/python-bin/python3" "$base_dir/ninja.py" "$@"

ninja.bat

@@ -4,9 +4,11 @@
 :: found in the LICENSE file.
 setlocal
 
+set scriptdir=%~dp0
+
 :: Ensure that "depot_tools" is somewhere in PATH so this tool can be used
 :: standalone, but allow other PATH manipulations to take priority.
-set PATH=%PATH%;%~dp0
+set PATH=%PATH%;%scriptdir%
 
 :: Defer control.
-python3 "%~dp0\ninja.py" %*
+call %scriptdir%python-bin\python3.bat "%~dp0\ninja.py" %*