cmd/libexpr: don't determinePos in value-to-json

it's not useful. nix-instantiate will always point to the wrong location
because "at the toplevel" is not a position we can point to. all nesting
conversions will point to strange places: the *original* definition of a
lambda value (or partial applications thereof), or nowhere at all (since
only lambda values can have their positions determined in the formatter)

Change-Id: I56bd9e1cf3cae11b4198f148a141ca758e4fe8ac

tests/functional/lang/eval-fail-toJSON-nested.err.exp

@@ -0,0 +1,18 @@
+error:
+       … while calling the 'toJSON' builtin
+         at /pwd/lang/eval-fail-toJSON-nested.nix:1:1:
+            1| builtins.toJSON { foo = a: 1; }
+             | ^
+            2|
+
+       … while evaluating attribute 'foo'
+         at /pwd/lang/eval-fail-toJSON-nested.nix:1:19:
+            1| builtins.toJSON { foo = a: 1; }
+             |                   ^
+            2|
+
+       error: cannot convert a function to JSON
+       at /pwd/lang/eval-fail-toJSON-nested.nix:1:19:
+            1| builtins.toJSON { foo = a: 1; }
+             |                   ^
+            2|