bin/brew: don't allow unbound variables.

This can fail for users using `bash -u` or `set -u` in their shell so
let's try to be stricter here for both them and us.

bin/brew

@@ -1,4 +1,4 @@
-#!/bin/bash -p
+#!/bin/bash -pu
 set -u
 
 # Fail fast with concise message when not using bash