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/test-systemctl-enable.sh

@@ -3,7 +3,7 @@
 set -ex
 
 # Silence warning from running_in_chroot_or_offline()
-export SYSTEMD_IGNORE_CHROOT=1
+export SYSTEMD_IN_CHROOT=0
 
 systemctl=${1:-systemctl}
 systemd_id128=${2:-systemd-id128}

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)