Fixed Bash script header to dynamically load correct environment.

- Improves loading of the correct Bash environment/version, especially
  on systems that might not be using a standard install (i.e.
  Homebrew), by loading a version of Bash as defined by the environment
  rather than a hard coded path.

[ci skip]

bin/create_boot_disk

@@ -1,4 +1,4 @@
-#! /bin/bash
+#! /usr/bin/env bash
 
 # DESCRIPTION
 # Creates macOS boot disk.