Use #!/usr/bin/env for bash shebangs.

/bin/sh and /usr/bin/env are the only two binaries that NixOS provides
at a fixed path (outside a buildFHSUserEnv sandbox).

This discussion was split from #11004.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

scripts/upgrade-zulip

@@ -1,2 +1,2 @@
-#!/bin/bash
+#!/usr/bin/env bash
 "$(dirname "$0")/lib/upgrade-zulip" "$@" 2>&1 | tee -a /var/log/zulip/upgrade.log