Require flakes for the -I flake:... feature

src/libcmd/common-eval-args.cc

@@ -161,6 +161,7 @@ SourcePath lookupFileArg(EvalState & state, std::string_view s)
     }
 
     else if (hasPrefix(s, "flake:")) {
+        settings.requireExperimentalFeature(Xp::Flakes);
         auto flakeRef = parseFlakeRef(std::string(s.substr(6)), {}, true, false);
         auto [accessor, _] = flakeRef.resolve(state.store).lazyFetch(state.store);
         return accessor->root();

src/libexpr/flake/flake.cc

@@ -698,6 +698,8 @@ void callFlake(EvalState & state,
     const LockedFlake & lockedFlake,
     Value & vRes)
 {
+    settings.requireExperimentalFeature(Xp::Flakes);
+
     auto [lockFileStr, keyMap] = lockedFlake.lockFile.to_string();
 
     auto overrides = state.buildBindings(lockedFlake.nodePaths.size());

src/libexpr/parser.y

@@ -805,6 +805,7 @@ std::optional<SourcePath> EvalState::resolveSearchPathElem(const SearchPathElem
     }
 
     else if (hasPrefix(elem.second, "flake:")) {
+        settings.requireExperimentalFeature(Xp::Flakes);
         auto flakeRef = parseFlakeRef(elem.second.substr(6), {}, true, false);
         debug("fetching flake search path element '%s''", elem.second);
         auto [accessor, _] = flakeRef.resolve(store).lazyFetch(store);