Start a SSH agent in the recipe to allow cargo to fetch crates from SSH git repos

[igiona]

Since we do not have a rust-crate repository, we link our own libraries via SSH git repositories.

Currently, we need to start a ssh-agent in our pipeline in order to be able to compile a rust application using this recipe.
It would be nice if the recipe itself would take care of it, as well as the additional side-effects/issues (see #166).