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>

tools/deploy-branch

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 function error_out {
     echo -en '\e[0;31m'