lib/strings: forbid lists in isStorePath

When a list is passed to isStorePath this is most likely a mistake and
it is therefore better to just return false. There is one case where
this theoretically makes sense (if a list contains a single element for
which isStorePath elem), but since that case is also probably seldomly
intentional, it may save someone from debbuging unclear evaluation
errors.

lib/strings.nix

@@ -667,7 +667,7 @@ rec {
        => false
   */
   isStorePath = x:
-    if isCoercibleToString x then
+    if !(isList x) && isCoercibleToString x then
       let str = toString x; in
       substring 0 1 str == "/"
       && dirOf str == storeDir