Fix include and be more flexible for the script interpreter. (#1420)

Since we include now a test-infra script, which is Bash 4 based,
macOs user need to install Bash >= 4 e.g. via brew now.
The bash version coming with macOs is not good enough (3)

hack/build.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # Copyright 2018 The Knative Authors
 #
@@ -14,8 +14,6 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-source $(dirname $0)/../vendor/knative.dev/hack/library.sh
-
 set -o pipefail
 
 source_dirs="cmd pkg test lib tools"
@@ -347,6 +345,9 @@ if $(has_flag --debug); then
     set -x
 fi
 
+# Shared funcs from hack repo
+source $(basedir)/vendor/knative.dev/hack/library.sh
+
 # Shared funcs with CI
 source $(basedir)/hack/build-flags.sh