Regression in enabled_if for executables

With master, using `enabled_if` if executable stanza will lead to 

```
$ dune build
File "..."
3 |  (enabled_if (= %{os_type} "Unix"))
                      ^^^^^^^^
Error: This variable is only available since version 2.6 of the dune
language. Please update your dune-project file to have (lang dune 2.6).
```

This seems to be a regression introduced in #3408 