ci: update Kokoro build config

.kokoro/build.sh

@@ -16,7 +16,7 @@
 
 set -eo pipefail
 
-cd github/synthtool
+cd "$(dirname "$0")/.."
 
 # Disable buffering, so that the logs stream through.
 export PYTHONUNBUFFERED=1
@@ -25,4 +25,4 @@ export PYTHONUNBUFFERED=1
 nox -s lint test
 
 # remove all files, preventing kokoro from trying to sync them.
-rm -rf *
+rm -rf *

.kokoro/common.cfg

@@ -1,19 +1,7 @@
 # Format: //devtools/kokoro/config/proto/build.proto
 
-# Download trampoline resources.
-gfile_resources: "/bigstore/cloud-devrel-kokoro-resources/trampoline"
+build_file: "github/build-dir/.kokoro/build.sh"
 
-# Use the trampoline script to run in docker.
-build_file: "synthtool/.kokoro/trampoline.sh"
-
-# Configure the docker image for kokoro-trampoline.
-env_vars: {
-    key: "TRAMPOLINE_IMAGE"
-    value: "gcr.io/cloud-devrel-kokoro-resources/python"
-}
-
-# Tell the trampoline which build file to use.
-env_vars: {
-    key: "TRAMPOLINE_BUILD_FILE"
-    value: "github/synthtool/.kokoro/build.sh"
-}
+container_properties {
+  docker_image: "us-central1-docker.pkg.dev/client-library-releases/release-images-prod/python-multi@sha256:7cbcd29934e457c5f8b146b31026e786e6e3f9e182fcbb00ab52331202b9bb9a"
+}