bin/brew: unset POSIX mode.

We're using /bin/bash and not /bin/sh for a reason so don't allow users
to override this (seen in Homebrew/homebrew#49514).

Closes Homebrew/homebrew#49515.

bin/brew

@@ -1,4 +1,6 @@
 #!/bin/bash
+set +o posix
+
 chdir() {
   cd "$@" >/dev/null
 }