Rollup merge of #120203 - rowan-sl:usr-bin-env-bash, r=compiler-errors

Replace `#!/bin/bash` with `#!/usr/bin/env bash` in rust-installer tests

This allows the rust-installer tests to pass on NixOS

This change has [already been made](rust-lang/rust@302ad21) for the actual installer, it appears that the tests were just forgotten.

Author: matthiaskrgr