autoninja: Use bundled Python to invoke autoninja.py

`python-bin/python3` sets the depot_tool's Python binary dir to PATH.
This allows build steps to use the bundled Python, so that the Python version can be aligned with what the infra uses.
Note that autoninja.bat already uses the bundled Python.

Change-Id: I9350859bcc5bd0ed59f2f00be8e5615d89281daa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6395717
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Junji Watanabe <jwata@google.com>
Reviewed-by: Fumitoshi Ukai <ukai@google.com>
Auto-Submit: Junji Watanabe <jwata@google.com>

autoninja

@@ -14,13 +14,16 @@ if [ "$NINJA_SUMMARIZE_BUILD" == "1" ]; then
     export NINJA_STATUS="[%r processes, %f/%t @ %o/s : %es ] "
 fi
 
+scriptdir=$(dirname -- "$0")
+python_path="${scriptdir}/python-bin/python3"
+
 # Execute whatever is printed by autoninja.py.
 # Also print it to reassure that the right settings are being used.
-python3 "$(dirname -- "$0")/autoninja.py" "$@"
+"${python_path}" "${scriptdir}/autoninja.py" "$@"
 retval=$?
 
 if [ "$retval" == "0" ] && [ "$NINJA_SUMMARIZE_BUILD" == "1" ]; then
-  python3 "$(dirname -- "$0")/post_build_ninja_summary.py" "$@"
+  "${python_path}" "${scriptdir}//post_build_ninja_summary.py" "$@"
 fi
 
 # Pass-through autoninja's error code so that if a developer types: