myriad clone instructions : validate ssh-add

[gmanchon]

We had a setup issue where the myriad instructions to clone a challenge were failing because the student failed to correctly add their ssh key. The git push command was not returning any error but taking infinite time.

The solution used was to add ssh-agent to the list of oh-my-zsh plugins.

But an easier solution would be to verify at the end of the setup that the student has a correct configuration :

• bad conf :

➜  repo git:(master) ssh-add -l
The agent has no identities.

• good conf :

➜  repo git:(master) ssh-add -l
256 SHA256:xxxxxxxxxxxxxxxxxxxxxxxxx gmanchon@gmail.com (ED25519)