This was causing an issue in rust-analyzer, and probably would fail
in other ways via cargo. In any case it wasn't a good idea.
Signed-off-by: Alexis Sellier <alexis@radicle.xyz>
In interactive (user) contexts, use `try_sign` instead of `sign`.
We also revisit the `Profile` by adding a `signer` method that can
fail, eg. if we couldn't connect to ssh-agent.
Finally, we replace `UnsafeSigner` with `MemorySigner`, which makes
use of `ssh::Keystore`.
Signed-off-by: Alexis Sellier <alexis@radicle.xyz>