mkosi: /bin/bash -> /usr/bin/env bash

mkosi: /bin/bash -> /usr/bin/env bash

This is analogous to a change in mkosi[1] with the same motivation: some
scripts are run to bootstrap the distribution tree and are thus running
directly on the host system which may not have `/bin/bash` (e.g. NixOS).

As with the `mkosi` change, do that for each shebang for consistency
reasons.

[1] https://github.com/systemd/mkosi/commit/8d2cd50e5192cada7211724ecff3514f0c85cf0f