Change all fixed-path bash shebangs to /u/b/env bash outside test/

.github/workflows/build_test.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # SPDX-License-Identifier: LGPL-2.1-or-later
 
 set -ex

.github/workflows/run_mkosi.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # SPDX-License-Identifier: LGPL-2.1-or-later
 # shellcheck disable=SC2064
 

.github/workflows/unit_tests.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # SPDX-License-Identifier: LGPL-2.1-or-later
 
 # shellcheck disable=SC2206