libexpr: Replace Value::mkAttrs with constructor calls

The pseudo-constructor `Value::mkAttrs` would previously be called on an
default-constructed (under-initialized) `Value` to create a properly
initialized `Value` that represents an attribute set. This change
removes mkAttrs and constructs attr `Value`s directly.

Change-Id: I11cd801eefecd454a7a5b6229d6770d06a6a6964

lix/libexpr/primops/fetchTree.cc

@@ -68,7 +68,7 @@ void emitTreeAttrs(
             fmt("%s", std::put_time(std::gmtime(&*lastModified), "%Y%m%d%H%M%S")));
     }
 
-    v.mkAttrs(attrs);
+    v = {NewValueAs::attrs, attrs};
 }
 
 std::string fixURI(std::string uri, EvalState & state, const std::string & defaultScheme = "file")