Support NixOS and others non-FHS Linux distros

As it expects Bash to be at `/bin/bash`, the current implementation
fails to execute on NixOS, given that there bash is in a non-standard
path (i.g. `/nix/store/<hash>-bash-interactive-<version>/bin/bash`).

This patch specifically changes `/bin/bash` paths to use `/usr/bin/env bash`,
which should give the correct path in every Unix system.

Signed-off-by: Luis Holanda <luiscmholanda@gmail.com>

test/c/test.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 set -euo pipefail
 

toolchain/defs.bzl

@@ -76,7 +76,7 @@ def toolchains(
         },
     )
 
-ZIG_TOOL_WRAPPER = """#!/bin/bash
+ZIG_TOOL_WRAPPER = """#!/usr/bin/env bash
 set -e
 
 if [[ -n "$TMPDIR" ]]; then