executable_directory fails on Windows

I get the following error when trying to run one of my cross-platform justfiles on Windows:

```bash
just mise::install
error: Call to function `executable_directory` failed: executable directory not found
——▶ justfile-mise.just:25:17
│
25 │ mise_bin_dir := executable_directory()
│ ^^^^^^^^^^^^^^^^^^^^
error: Recipe `_update-mise` failed on line 21 with exit code 1
```

This is to be expected, since the underlying crate that provides that functionality indicates that it has no windows equivalent: https://docs.rs/dirs/latest/dirs/fn.executable_dir.html

My experience with the built-in just functions are very positive, and I hardly ever have to worry about whether a specific function is going to cause issues for developers on other platforms, so I was surprised when this issue occurred.

I can't think of a good solution to this issue, but perhaps it can be helpful to mention in the documentation that this specific function does not work on Windows.

As a side note, perhaps there can be a kind of "lint" command for just, that can report whether you are using functions in your justfiles that are known to not be "portable".