libexpr: throw a more helpful eval-error if a builtin is not available due to a missing feature-flag

I found it somewhat confusing to have an error like

    error: attribute 'getFlake' missing

if the required experimental-feature (`flakes`) is not enabled. Instead,
I'd expect Nix to throw an error just like it's the case when using e.g. `nix
flake` without `flakes` being enabled.

With this change, the error looks like this:

    $ nix-instantiate -E 'builtins.getFlake "nixpkgs"'
    error: Cannot call 'builtins.getFlake' because experimental Nix feature 'flakes' is disabled. You can enable it via '--extra-experimental-features flakes'.

           at «string»:1:1:

                1| builtins.getFlake "nixpkgs"
                 | ^

I didn't use `settings.requireExperimentalFeature` here on purpose
because this doesn't contain a position. Also, it doesn't seem as if we
need to catch the error and check for the missing feature here since
this already happens at evaluation time.

src/libexpr/eval.cc

@@ -465,6 +465,23 @@ EvalState::~EvalState()
 }
 
 
+void EvalState::requireExperimentalFeatureOnEvaluation(
+    const std::string & feature,
+    const std::string_view fName,
+    const Pos & pos)
+{
+    if (!settings.isExperimentalFeatureEnabled(feature)) {
+        throw EvalError({
+            .msg = hintfmt(
+                "Cannot call '%2%' because experimental Nix feature '%1%' is disabled. You can enable it via '--extra-experimental-features %1%'.",
+                feature,
+                fName
+            ),
+            .errPos = pos
+        });
+    }
+}
+
 Path EvalState::checkSourcePath(const Path & path_)
 {
     if (!allowedPaths) return path_;