Add $SYSTEMD_IN_CHROOT to override chroot detection

When running unprivileged, checking /proc/1/root doesn't work because
it requires privileges. Instead, let's add an environment variable so
the process that chroot's can tell (systemd) subprocesses whether
they're running in a chroot or not.

test/units/TEST-74-AUX-UTILS.detect-virt.sh

@@ -0,0 +1,7 @@
+#!/usr/bin/env bash
+# SPDX-License-Identifier: LGPL-2.1-or-later
+set -eux
+set -o pipefail
+
+SYSTEMD_IN_CHROOT=1 systemd-detect-virt --chroot
+(! SYSTEMD_IN_CHROOT=0 systemd-detect-virt --chroot)