ci: fix shebang in build-deqp-runner.sh

We use bash features such as `pushd`, so be accurate in the shebang,
even though in practice this is sourced not executed so this shebang
only serves our editors.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25430>

.gitlab-ci/container/build-deqp-runner.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/usr/bin/env bash
 # shellcheck disable=SC2086 # we want word splitting
 
 set -ex