fixup! [OCaml] add prefix

ocaml.groovy

@@ -36,7 +36,7 @@ node('master') {
     try {
         stage 'Checkout'
         checkout scm
-        sh 'git rev-parse --short HEAD | GIT_HEAD'
+        sh 'git rev-parse --short HEAD > GIT_HEAD'
         commit = readFile('GIT_HEAD')
         prefix = "${prefix}${commit}"
         echo "${prefix}"