[GR-19220] Support sourcing the Spin environment file from a Bash script (#2373)

PullRequest: truffleruby/2713

Author: eregon

.spin/bin/env

@@ -1,6 +1,5 @@
-#!/bin/bash
-
-SCRIPT_PATH=$(dirname "$0")
+# $BASH_SOURCE[0] is for Bash, ${(%):-%x} is for Zsh
+SCRIPT_PATH=$(dirname "${BASH_SOURCE[0]:-${(%):-%x}}")
 SCRIPT_PATH=$(cd "$SCRIPT_PATH" && pwd)
 TRUFFLERUBY_DIR=$SCRIPT_PATH/../..
 export JT_ENV=jvm-ce