Change default executable name when on windows

[Rafaeruo]

This changes the default behavior of EvaluatorManager when no pkl executable path is provided.

We used to default to the executable name pkl, which was our best guess of the name of pkl's executable in the user's PATH.

When on Windows, I think it would make more sense to add the .exe file extension to the file name: pkl.exe. Although we can't be sure that the user's PATH will contain an executable named exactly that way, it's serves as a much better guess for the platform.

[jeremyvaartjes]

Not sure that we need to optimise to this level but this might be a more "performant" version of the function

public static string DefaultPklExecutableName
#if TARGET_WINDOWS
    => "pkl.exe";
#else
    => "pkl";
#endif

I'm guessing this PR was triggered by the failing github actions builds. This is due to an issue with the setup pkl build step and not because of the .exe being dropped from our dotnet code. I've actually put in a fix here: pkl-community/setup-pkl#41 but the issue is that github is still pointing to v0.0.5 and not what is currently in the master branch. I might ping the lead dev of that repo to see if we can get a v0.0.6 to be put together.

[jeremyvaartjes]

Happy with this code either way. Feel free to use my comment above or ignore it, I don't think it makes much difference tbh.

[Rafaeruo]

I'm guessing this PR was triggered by the failing github actions builds.

That is correct.

but the issue is that github is still pointing to v0.0.5 and not what is currently in the master branch

I did notice that, but I also assumed that fixing the issue would require both updating pkl-setup and making this change in the way we call pkl in our code. That assumption was incorrect, though: apparently Windows allows omitting the file extension when running programs.