Use /usr/bin/env bash instead of /bin/bash for shebang

Some systems like NixOS don't have `/bin/bash`.
Ref: https://discourse.nixos.org/t/add-bin-bash-to-avoid-unnecessary-pain/5673

This changes allow to run scripts in those systems.

Fixes #43.

scripts/setup-plan.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # Setup implementation plan structure for current branch
 # Returns paths needed for implementation plan generation
 # Usage: ./setup-plan.sh [--json]