treewide: more portable bash shebangs

As in 2a5fcfae02
and in 3e67e5c992
using /usr/bin/env allows bash to be looked up in PATH
rather than being hard-coded.

As with the previous changes the same arguments apply
- distributions have scripts to rewrite shebangs on installation and
  they know what locations to rely on.
- For tests/compilation we should rather rely on the user to have setup
  there PATH correctly.

In particular this makes testing from git easier on NixOS where do not provide
/bin/bash to improve compose-ability.

src/basic/linux/update.sh

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