SSH_AGENT_PID not set

`SSH_AGENT_PID` is not set when I try to add an SSH key with `ssh-add ~/.ssh/id_rsa`.

```
$ ssh-add ~/.ssh/id_rsa
Could not open a connection to your authentication agent.
$ echo $SSH_AGENT_PID

$ # ↑ empty string ↑
```

Yet, `ssh-agent` is well started:

```
$ ps aux | grep ssh-agent
morgan 16643 0.0 0.2 3688 984 ? Ss 11:43 0:00 ssh-agent -c
```

I did manage to add a key with the following commands:

```
$ eval (ssh-agent -c)
Agent pid 18541
$ echo $SSH_AGENT_PID
18541
$ ssh-add ~/.ssh/id_rsa
Identity added: /home/morgan/.ssh/id_rsa (rsa w/o comment)
```

My configuration:

```
$ fish --version
fish, version 2.2.0
$ ssh -V
OpenSSH_6.7p1 Raspbian-5+deb8u3, OpenSSL 1.0.1t 3 May 2016
```

Not sure how `ssh-agent` works though... Any help would be much appreciated.

No file or test is named. Start by reproducing the fish 2.2.0 commands with ssh-agent -c, ssh-add, and the reported environment, then trace how the fish-ssh-agent entry point exports SSH_AGENT_PID; done means ssh-add can connect to the running agent without manual recovery commands.

Written by the indexing model from the issue text.