Fix isString to refrain from coercing values

Derivations can be automatically coerced to strings. They are however
not strings and `isString` should return `False` on them.

src/Nix/Builtins.hs

@@ -1040,10 +1040,6 @@ isList
   :: forall e t f m . MonadNix e t f m => NValue t f m -> m (NValue t f m)
 isList = hasKind @[NValue t f m]
 
-isString
-  :: forall e t f m . MonadNix e t f m => NValue t f m -> m (NValue t f m)
-isString = hasKind @NixString
-
 isInt
   :: forall e t f m . MonadNix e t f m => NValue t f m -> m (NValue t f m)
 isInt = hasKind @Int
@@ -1060,6 +1056,12 @@ isNull
   :: forall e t f m . MonadNix e t f m => NValue t f m -> m (NValue t f m)
 isNull = hasKind @()
 
+-- isString cannot use `hasKind` because it coerces derivations to strings.
+isString :: MonadNix e t f m => NValue t f m -> m (NValue t f m)
+isString v = demand v $ \case
+  NVStr{} -> toValue True
+  _       -> toValue False
+
 isFunction :: MonadNix e t f m => NValue t f m -> m (NValue t f m)
 isFunction func = demand func $ \case
   NVClosure{} -> toValue True