Make script/ entrypoints POSIX sh; add bootstrap and setup

script/check

@@ -1,9 +1,9 @@
-#!/usr/bin/env bash
+#!/bin/sh
 # script/check: run all checks (test, lint, fmt-check). Our own
 # extension to scripts-to-rule-them-all. Must not modify any files.
-set -euo pipefail
+set -eu
 
-SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd -P)"
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
 
 main() {
     "$SCRIPT_DIR/test"