Fix kythe build.

* Pass --auth_credentials now that we're using RBE
* Fix kzips not being uploaded to the root of the GCS bucket

PiperOrigin-RevId: 291241757

kokoro/kythe/generate_xrefs.cfg

@@ -23,6 +23,7 @@ bazel_setting {
 
 action {
   define_artifacts {
-    regex: "*.kzip"
+    regex: "**/*.kzip"
+    fail_if_no_artifacts: true
   }
 }

kokoro/kythe/generate_xrefs.sh

@@ -48,6 +48,7 @@ bazel \
   --define=kythe_corpus=gvisor.dev \
   --cxxopt=-std=c++17 \
   --config=remote \
+  --auth_credentials="${KOKORO_BAZEL_AUTH_CREDENTIAL}" \
   //...
 
 "${KYTHE_DIR}/tools/kzip" merge \