Consistently use /usr/bin/env in script shebangs

#!/bin/bash is used consistently throughout the project, but we should coalesce around the better option of #!/usr/bin/env bash

It would be even better if we could add a gradle task to ensure this for all files in the project, regardless of if the file is a bash script or not. We should ensure that #!/usr/bin/env is used for all shebangs throughout the project.