Upgrade Bazel before trying to build with it.

Change-Id: I412f23a7e4e933642d6fecdededca4aea0190c37
Reviewed-on: https://code-review.googlesource.com/c/re2/+/44214
Reviewed-by: Paul Wankadia <junyer@google.com>
diff --git a/kokoro/bazel.sh b/kokoro/bazel.sh
index 95aee2e..7437ef3 100755
--- a/kokoro/bazel.sh
+++ b/kokoro/bazel.sh
@@ -3,6 +3,16 @@
 
 cd git/re2
 
+case "${KOKORO_JOB_NAME}" in
+  */windows-*)
+    choco upgrade bazel -y
+    ;;
+  *)
+    # Use the script provided by Kokoro.
+    use_bazel.sh latest
+    ;;
+esac
+
 bazel clean
 bazel build --compilation_mode=dbg -- //:all
 bazel test  --compilation_mode=dbg --test_output=errors -- //:all \