replace /usr/bin/env sh with /bin/sh

/bin/sh exists in NixOS and in fact is the only file in `/bin`:

    $ docker run -ti --rm nixos/nix ls /bin /usr/bin
    /bin:
    sh

    /usr/bin:
    env

More context in
https://lists.sr.ht/~motiejus/bazel-zig-cc/%3CCAFVMu-pvNx%2BpeQYdge_fvmSGrHDmn78VmoYwTxkDgMjbzfVAwQ%40mail.gmail.com%3E

bin/mod-tidy

@@ -1,6 +1,5 @@
-#!/usr/bin/env bash
-
-set -xeuo pipefail
+#!/bin/sh
+set -eu
 
 cd "$(git rev-parse --show-toplevel)/"
 bazel run @go_sdk//:bin/go -- mod tidy