Move buildkite's SSH key out of /home/wpcarro into /etc/ssh

After enabling buildkite-agent using NixOS, it runs as its own user,
buildkite-agent-socrates, which does not have its own home directory. I moved
the SSH key that I made when running buildkite-agent as wpcarro into /etc/ssh
and `chown`'d it for buildkite-agent-socrates.

nixos/socrates/default.nix

@@ -112,6 +112,7 @@ in {
     socrates = {
       enable = true;
       tokenPath = "/etc/secrets/buildkite-agent-token";
+      privateSshKeyPath = "/etc/ssh/buildkite_agent_id_rsa";
     };
   };