Fix Dotty classpath

[travisbrown]

depsClasspath is already a classpath string, so interpolating : results in classpath arguments like this 😄:

/:h:o:m:e:/:t:r:a:v:i:s:/:.:c:o:m:p:i:l:e:r:B:e:n:c:h:m:a:r:k:/:d:e:p:s:/:c:a:t:s:-:k:e:r:n:e:l:_:2:.:1:3:-:2:.:1:.:1:.:j:a:r

I've also bumped dottyLatest and the sbt-dotty version, but would be happy to split that out into a separate PR or drop it.

[smarter]

LGTM but I don't have merge rights here /cc @retronym

[retronym]

Thanks!