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.

src/basic/virt.c

@@ -896,6 +896,13 @@ int running_in_chroot(void) {
          * mount /proc, so all other programs can assume that if /proc is *not* available, we're in some
          * chroot. */
 
+        r = getenv_bool("SYSTEMD_IN_CHROOT");
+        if (r >= 0)
+                return r > 0;
+        if (r != -ENXIO)
+                log_debug_errno(r, "Failed to parse $SYSTEMD_IN_CHROOT, ignoring: %m");
+
+        /* Deprecated but kept for backwards compatibility. */
         if (getenv_bool("SYSTEMD_IGNORE_CHROOT") > 0)
                 return 0;