lib/trivial: drop rec in favor of lib fixpoint

That way each expression uses the final version of other lib.trivial
declarations.

For instance, when replacing `versionSuffix` with the string `"fnord"`
in a lib overlay, `trivial.version` uses `"fnord"` as suffix now rather
than `pre-git`.

lib/trivial.nix

@@ -1,6 +1,18 @@
 { lib }:
 
-rec {
+let
+  inherit (lib.trivial)
+    isFunction
+    isInt
+    functionArgs
+    pathExists
+    release
+    setFunctionArgs
+    toBaseDigits
+    version
+    versionSuffix
+    warn;
+in {
 
   ## Simple (higher order) functions
 
@@ -439,7 +451,7 @@ rec {
   */
   functionArgs = f:
     if f ? __functor
-    then f.__functionArgs or (lib.functionArgs (f.__functor f))
+    then f.__functionArgs or (functionArgs (f.__functor f))
     else builtins.functionArgs f;
 
   /* Check whether something is a function or something