Install sdkman after the switch, using bash from Nix

Install sdkman after the switch, using bash from Nix sdkman's installer requires Bash 4+ and macOS ships 3.2.57, so it aborted on a fresh bootstrap. Adds bash (5.3p15) to the package set rather than taking a new Homebrew dependency for it. That creates an ordering constraint the original script had backwards: sdkman ran as step 3, before the switch that provides bash. The switch and sdkman steps are swapped, and the sdkman step now checks for the interpreter and skips with an explanation rather than failing the whole script if it is missing. Adding bash to the user profile means scripts using `#!/usr/bin/env bash` resolve to 5.3 rather than the system 3.2. That is the point, but it is a behaviour change worth knowing about; `#!/bin/bash` scripts are unaffected.