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

ci/test

@@ -1,2 +1,2 @@
-#!/usr/bin/env sh
+#!/bin/sh
 exec bazel test "$@" ...

test/c/test.sh

@@ -1,6 +1,5 @@
-#!/usr/bin/env bash
-
-set -euo pipefail
+#!/bin/sh
+set -eu
 
 # shellcheck disable=SC2153
 want=$WANT

toolchain/defs.bzl

@@ -103,13 +103,13 @@ set ZIG_GLOBAL_CACHE_DIR=%ZIG_LOCAL_CACHE_DIR%
 "{zig}" "{zig_tool}" %*
 """
 
-_ZIG_TOOL_WRAPPER_CACHE_KNOWN = """#!/usr/bin/env sh
+_ZIG_TOOL_WRAPPER_CACHE_KNOWN = """#!/bin/sh
 export ZIG_LOCAL_CACHE_DIR="{cache_prefix}/bazel-zig-cc"
 export ZIG_GLOBAL_CACHE_DIR="{cache_prefix}/bazel-zig-cc"
 exec "{zig}" "{zig_tool}" "$@"
 """
 
-_ZIG_TOOL_WRAPPER_CACHE_GUESS = """#!/usr/bin/env sh
+_ZIG_TOOL_WRAPPER_CACHE_GUESS = """#!/bin/sh
 set -e
 if [ -n "$TMPDIR" ]; then
     _cache_prefix=$TMPDIR