build: Read the manifest.xml from the repo location

In our original world, we were checking out manifests in a dedicated repository.
In the new repo world, we should use the .repo path.

Change-Id: I9ea27d3bf6e01abdf184255705ffb6dde7a93d66
diff --git a/kokoro/build.sh b/kokoro/build.sh
index 11146c3..819a74e 100644
--- a/kokoro/build.sh
+++ b/kokoro/build.sh
@@ -47,7 +47,7 @@
 
 pushd ${ROOTDIR}
 python3 ${ROOTDIR}/build/create_release_manifest.py \
-  -i ${ROOTDIR}/manifest/default.xml \
+  -i ${ROOTDIR}/.repo/manifest.xml \
   -o ${PRODUCT_OUT}/manifest.xml
 popd