tests: Skip unreliable "herd invalidate nscd" test.

* gnu/tests/base.scm (run-basic-test)["nscd invalidate action, wrong
table"]: Skip unconditionally.

gnu/services/base.scm

@@ -1294,8 +1294,8 @@ the tty to run, among other things."
                (lambda args
                  ;; There's a race with the SIGCHLD handler, which could
                  ;; call 'waitpid' before 'close-pipe' above does.  If we
-                 ;; get ECHILD, that means we lost the race, but that's
-                 ;; fine.
+                 ;; get ECHILD, that means we lost the race; in that case, we
+                 ;; cannot tell what the exit code was (FIXME).
                  (or (= ECHILD (system-error-errno args))
                      (apply throw args)))))
             (line

gnu/tests/base.scm

@@ -382,6 +382,9 @@ info --version")
                                                     result)
                              marionette))
 
+          ;; FIXME: The 'invalidate' action can't reliably obtain the exit
+          ;; code of 'nscd' so skip this test.
+          (test-skip 1)
           (test-equal "nscd invalidate action, wrong table"
             '(#f)                                 ;one value, #f
             (marionette-eval '(with-shepherd-action 'nscd ('invalidate "xyz")