Use env for bash path (#8716)

Fixes source building under FreeBSD where a symlink has not been made for bash

Context
/bin/bash is standard under OSes that ship with bash installed by default. FreeBSD does not have bash as a default shell.

Changes Made
For portability /usr/bin/env bash is used on the shebang line. This also bring it inline with other dotnet repos.

Testing
none needed?

Notes
env is standard at this point. I have been unable to find a non-historic UNIX-like system that places env in a location exclusively that is not /usr/bin/

build.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 SOURCE="${BASH_SOURCE[0]}"
 while [ -h "$SOURCE" ]; do # resolve $SOURCE until the file is no longer a symlink