Don't override KUBE_RELEASE_RUN_TESTS if set in enviroment

hack/jenkins/build.sh

@@ -31,9 +31,11 @@ set -o xtrace
 # space.
 export HOME=${WORKSPACE} # Nothing should want Jenkins $HOME
 export PATH=$PATH:/usr/local/go/bin
-export KUBE_RELEASE_RUN_TESTS=n
 export KUBE_SKIP_CONFIRMATIONS=y
 
+: ${KUBE_RELEASE_RUN_TESTS:="n"}
+export KUBE_RELEASE_RUN_TESTS
+
 # Clean stuff out. Assume the last build left the tree in an odd
 # state.
 rm -rf ~/.kube*