buildscripts: Specify Bazel version to use

The version Kokoro uses changes over time. We need a stable version so
our build doesn't break when Bazel is upgraded.
diff --git a/buildscripts/kokoro/bazel.sh b/buildscripts/kokoro/bazel.sh
index 13bbaa4..0dd1d75 100755
--- a/buildscripts/kokoro/bazel.sh
+++ b/buildscripts/kokoro/bazel.sh
@@ -3,6 +3,8 @@
 set -exu -o pipefail
 cat /VERSION
 
+use_bazel.sh 0.12.0
+
 cd github/grpc-java
 bazel build ...