Unrolled build for rust-lang#120203

Rollup merge of rust-lang#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@302ad21) for the actual installer, it appears that the tests were just forgotten.

Author: rust-timer

src/tools/rust-installer/test.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 set -e -u