autoninja: Generate UUID for AUTONINJA_BUILD_ID inside autoninja.py

Currently, {autoninja, autoninja.bat} scripts generate AUTONINJA_BUILD_ID by calling `python3 -c "import uuid; print(uuid.uuid4())"`.
This CL moves the logic at the beginning `autoninja.py`.

This also reduces the overhead of starting Python interpreter, which
takes around 60ms on my Windows workstaion.

```
> hyperfine python3
Benchmark 1: python3
  Time (mean ± σ):      63.4 ms ±   7.8 ms    [User: 25.7 ms, System: 33.7 ms]
  Range (min … max):    51.0 ms …  80.2 ms    41 runs
```

Change-Id: I93d5b65f3c5542c3a93a3de1f27a5aa5d06c09a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5737673
Reviewed-by: Fumitoshi Ukai <ukai@google.com>
Commit-Queue: Fumitoshi Ukai <ukai@google.com>
Auto-Submit: Junji Watanabe <jwata@google.com>
Commit-Queue: Junji Watanabe <jwata@google.com>

autoninja

@@ -10,10 +10,6 @@ if [ "$(expr "$(uname -s)" : "^MINGW64_NT")" == "10" ]; then
     exit
 fi
 
-# Set unique build ID if not already set by the user.
-AUTONINJA_BUILD_ID="${AUTONINJA_BUILD_ID:=$(python3 -c "import uuid; print(uuid.uuid4())")}"
-export AUTONINJA_BUILD_ID
-
 if [ "$NINJA_SUMMARIZE_BUILD" == "1" ]; then
     export NINJA_STATUS="[%r processes, %f/%t @ %o/s : %es ] "
 fi

autoninja.bat

@@ -13,11 +13,6 @@ if "%*" == "/?" (
   exit /b
 )
 
-if not defined AUTONINJA_BUILD_ID (
-  :: Set unique build ID.
-  FOR /f "usebackq tokens=*" %%a in (`%scriptdir%python-bin\python3.bat -c "import uuid; print(uuid.uuid4())"`) do set AUTONINJA_BUILD_ID=%%a
-)
-
 :: If a build performance summary has been requested then also set NINJA_STATUS
 :: to trigger more verbose status updates. In particular this makes it possible
 :: to see how quickly process creation is happening - often a critical clue on

autoninja.py

@@ -14,6 +14,7 @@ does handle import statements, but it can't handle conditional setting of build
 settings.
 """
 
+import uuid
 import logging
 import json
 import multiprocessing
@@ -407,6 +408,13 @@ def _upload_ninjalog(args):
 
 
 def main(args):
+    # Generate Build ID randomly.
+    # This ID is expected to be used consistently in all build tools.
+    build_id = os.environ.get("AUTONINJA_BUILD_ID")
+    if not build_id:
+        build_id = str(uuid.uuid4())
+        os.environ.setdefault("AUTONINJA_BUILD_ID", build_id)
+
     # Check the log collection opt-in/opt-out status, and display notice if necessary.
     should_collect_logs = build_telemetry.enabled()
     # On Windows the autoninja.bat script passes along the arguments enclosed in