kokoro linux: use 'lean' image based on ubuntu 24.04

BUG=391948942

kokoro/linux/common.cfg

@@ -0,0 +1,17 @@
+# Copyright (C) 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+env_vars {
+  key: "LINUX_BUILDER_IMAGE"
+  value: "us-east4-docker.pkg.dev/shaderc-build/radial-docker/radial-ubuntu-2404-amd64-lean"
+}