Don't require execute bit if --artifact and --invoker provided

[trisberg]

Currently there is no way to create a command function on Windows since there is no way to set the execute bit. If the user provides bot --artifact and --invoker I think the buildpack should accept the command and set the execute bit on the command file. This would allow the Windows user to run local-path builds from the PowerShell.

[scothis]

This feels like a more generic problem that buildpacks need to solve. Do they have a recommendation?

I'd to avoid automatically flipping that bit since if the target artifacts invokes another file without the bit that should have it, it will fail in an even more mysterious way.

[scothis]

Replaced by #10