Add a vendor check to CI

Signed-off-by: Daniel Nephin <dnephin@gmail.com>

script/validate/vendor

@@ -0,0 +1,18 @@
+#!/usr/bin/env bash
+set -eu -o pipefail
+
+vndr 2>&1 | grep -v -i clone
+
+DIFF_PATH="vendor/"
+DIFF=$(git status --porcelain -- "$DIFF_PATH")
+
+if [ "$DIFF" ]; then
+    echo
+    echo "These files were modified:"
+    echo
+    echo "$DIFF"
+    echo
+    exit 1
+else
+    echo "$DIFF_PATH is correct"
+fi