Add support for Ubuntu 20

[pascalberger]

LibGit2Sharp 0.26.x doesn't support Ubuntu 20. Since then there was an effort to replace native binaries with managed HTTP stack (libgit2/libgit2sharp#1618) which should bring support for more distros.

This is not available yet in a final release, but only in preview releases (https://www.nuget.org/packages/LibGit2Sharp/0.27.0-preview-0096). We should think about using a preview release in Cake.Git as it would increase usability of the addin quite a bit, and managed services or Docker images start to update to unsupported distros by default.

[justin-caldicott]

Funny that you only just created this issue as I just hit this in our Azure PIpeline builds. They upgraded the ubuntu-latest image to 20.04 recently.

I actually upgraded GitVersion first as that's where the internet pointed for a solution. However on looking more closely I see the error is from cake GitClone.

In the meantime I'll pin our ubuntu based builds to ubuntu-18.04.

[pascalberger]

@justin-caldicott Which version of Cake.Git are you using?

[justin-caldicott]

Originally 0.21.0 but also 1.0.1