…uilding unknown exception types

The exception constructors in GitErrorsToLibGit2SharpExceptions make use of the error category by passing it through to the NativeException(string message, GitErrorCategory category) constructor which adds ("libgit2.category", category) to Data on System.Exception.

We were calling new LibGit2SharpException(m, c), but this was resolving to the string formatting constructor on LibGit2SharpException, because it does not have a constructor that takes a category. This runs a string format, so if the errorMessage contained any curly braces, that constructor would throw an System.FormatException.

This has been changed to just use the message constructor (with no format arguments), and will drop the error code (which is always Unknown anyway).

…he subtransport

Re-introduce .net6 while we still have Calamari running in .net6