[autoninja] Respect user-provided AUTONINJA_BUILD_ID

Change-Id: I2e1b14827e7ff4c1e88a211f5ae0d7a3322b6f39
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3569027
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Yannic Bonenberger <yannic.bonenberger@gmail.com>

autoninja

@@ -10,8 +10,8 @@ if [ "$(expr "$(uname -s)" : "^MINGW64_NT")" == "10" ]; then
     exit
 fi
 
-# Set unique build ID.
-AUTONINJA_BUILD_ID="$(python3 -c "import uuid; print(uuid.uuid4())")"
+# Set unique build ID if not already set by the user.
+AUTONINJA_BUILD_ID="${AUTONINJA_BUILD_ID:=$(python3 -c "import uuid; print(uuid.uuid4())")}"
 export AUTONINJA_BUILD_ID
 
 if [ "$NINJA_SUMMARIZE_BUILD" == "1" ]; then