ci: unify bash calling convention

ci: unify bash calling convention

Under ci/ hierarchy, we run scripts under either "sh" (any Bourne
compatible POSIX shell would work) or specifically "bash" (as they
require features from bash, e.g., ${parameter/pattern/string}
expansion).  As we have the CI environment under our control, we can
expect that /bin/sh will always be fine to run the scripts that only
require a Bourne shell, but we may not know where "bash" is
installed depending on the distro used.

So let's make sure we start these scripts with either one of these:

	#!/bin/sh
	#!/usr/bin/env bash

Yes, the latter has to assume that everybody installs "env" at that
path and not as /bin/env or /usr/local/bin/env, but this currently
is the best we could do.

Signed-off-by: Junio C Hamano <gitster@pobox.com>