bootctl: don't load etc/machine-info from cwd

arg_root defaults to null, so if --root isn't given, this would try reading
etc/machine-info from the current working directory, which is likely to fail.

Fixes: 77db9ef ("boot: Make sure we take --root into account everywhere.")
(cherry picked from commit 0452779)
(cherry picked from commit 8d7eef9)
(cherry picked from commit a573fd1)

Author: alyssais

src/boot/bootctl-install.c

@@ -46,7 +46,7 @@ static int load_etc_machine_info(void) {
         _cleanup_free_ char *p = NULL, *s = NULL, *layout = NULL;
         int r;
 
-        p = path_join(arg_root, "etc/machine-info");
+        p = path_join(arg_root, "/etc/machine-info");
         if (!p)
                 return log_oom();