rootfs: Fix the paths to match up with the new kokoro build setup

Change-Id: I7060bc3c25ee85b227178f81e537b4744896df78
diff --git a/kokoro/rootfs.cfg b/kokoro/rootfs.cfg
index 216f446..84d429b 100644
--- a/kokoro/rootfs.cfg
+++ b/kokoro/rootfs.cfg
@@ -1 +1 @@
-build_file: "continuous-build/build/kokoro/rootfs.sh"
+build_file: "git/build/kokoro/rootfs.sh"