kokoro: Include kokoro image version in build logs

Otherwise there's no way to determine which version of the image was
used for the build.
diff --git a/buildscripts/kokoro/bazel.sh b/buildscripts/kokoro/bazel.sh
index 7a62be8..9fbeebb 100755
--- a/buildscripts/kokoro/bazel.sh
+++ b/buildscripts/kokoro/bazel.sh
@@ -1,6 +1,7 @@
 #!/bin/bash
 
 set -exu -o pipefail
+cat /VERSION
 
 cd github/grpc-java
 bazel build ...