list-runtime-roots: Ignore ESRCH while reading from /proc.

Fixes <https://bugs.gnu.org/29368>.
Reported by Martin Castillo <castilma@uni-bremen.de>.

* nix/scripts/list-runtime-roots.in (referenced-files): Ignore ESRCH.

nix/scripts/list-runtime-roots.in

@@ -121,6 +121,7 @@ or the empty list."
                     (lambda args
                       (let ((err (system-error-errno args)))
                         (if (or (= ENOENT err)    ;TOCTTOU race
+                                (= ESRCH err)     ;ditto
                                 (= EACCES err))   ;not running as root
                             '()
                             (apply throw args)))))))