libexpr: fix app chain extension

during the value rewrite we accidentally broke extension of incomplete
primop application. this only shows up when binding on incomplete call
to a primop to a name, binding an incomplete call to *that* to another
name, and then finally calling the second binding with enough args for
a complete primop application. since this only shows up when calling a
primop with three or more args it took a while to surface. we have few
builtins that match this: foldl', replaceStrings, and substring. these
are not used incompletely in this manner very often, so this lingered.

fixes #1102

Change-Id: I218dffc14ae876efc86a86c7eb6c895e2405201c

tests/functional2/lang/call-primop/eval-okay-app-extension.out.exp

@@ -0,0 +1 @@
+"234"

tests/functional2/lang/call-primop/in-app-extension.nix

@@ -0,0 +1,5 @@
+let
+  a = builtins.substring 1;
+  b = a 3;
+in
+b "1234567890"