fix: build with --output without trailing slash

[mxschmitt]

Fixes #2827

My first attempt was using OutDir but it broke some of the LocalNugetTests. Instead we now call EnsureTrailingSlash.

[campersau]

FYI: you used OutputDir instead of OutDir in your first commit: faf6ea0

[mxschmitt]

Great discovery! No strong feelings, both will probably work (OutDir vs. OutputPath). PublishDir includes a trailing slash already, but wrapping it does probably not hurt either.

[campersau]

In my testing PublishDir did not contain a trailing slash when a custom --output path was given which didn't include it #2827 (comment) . So I don't think that the MS documentation is correct in this case.
Either way it does not hurt to just always call EnsureTrailingSlash.