chore(bigframes): use trampoline as the build file for bigframes load test (#17160)

Tracking issue: b/509623551.

It seems the old build file no longer works with the setup of mono repo.

Author: sycai

.kokoro/build-bigframes.sh

@@ -0,0 +1,27 @@
+# Format: //devtools/kokoro/config/proto/build.proto
+
+# Build logs will be here
+action {
+  define_artifacts {
+    regex: "**/*sponge_log.xml"
+  }
+}
+
+# Download trampoline resources.
+gfile_resources: "/bigstore/cloud-devrel-kokoro-resources/trampoline"
+
+# Download resources for system tests (service account key, etc.)
+gfile_resources: "/bigstore/cloud-devrel-kokoro-resources/google-cloud-python"
+
+# Use the trampoline script to run in docker.
+build_file: "google-cloud-python/.kokoro/trampoline.sh"
+
+# Configure the docker image for kokoro-trampoline.
+env_vars: {
+    key: "TRAMPOLINE_IMAGE"
+    value: "gcr.io/cloud-devrel-kokoro-resources/python-multi"
+}
+env_vars: {
+    key: "TRAMPOLINE_BUILD_FILE"
+    value: "github/google-cloud-python/.kokoro/system.sh"
+}

.kokoro/load/common.cfg

@@ -1,13 +1,5 @@
 # Format: //devtools/kokoro/config/proto/build.proto
 
-# Build logs will be here
-action {
-  define_artifacts {
-    regex: "**/*sponge_log.xml"
-  }
-}
-
-build_file: "google-cloud-python/.kokoro/build-bigframes.sh"
 timeout_mins: 720
 
 # Only run this nox session.