Make bash scripts work for Bash v4+

[carlisia]

Some bash scripts in this repo have #!/bin/bash at the top of the file. They need to have #!/usr/bin/env bash so it will pick up updated versions of bash on MacOS.

It should have no other side effect.

For context: for virtually any knative script to be run, the bash version needs to be v4+. See knative/test-infra#2143 for background.

[carlisia]

/assign carlisia

[dprotaso]

closing since #634 merged