nixos/tests/gitolite: Don't build during evaluation

Noticed in https://hydra.nixos.org/jobset/nixos/release-17.09#tabs-errors:

````
hydra-eval-jobs returned exit code 1:
building path(s) '/nix/store/wxcbjli7m98yymnxrxkf6pigr7a05zad-id_ed25519.pub'
building '/nix/store/gyig2d7cry98647h0grfilq26cpc1wy8-id_ed25519.pub.drv'...
````

Issue #29774

nixos/tests/gitolite.nix

···
     -----END OPENSSH PRIVATE KEY-----
   '';
 
-  adminPublicKey = pkgs.writeText "id_ed25519.pub" ''
+  adminPublicKey = ''
     ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIO7urFhAA90BTpGuEHeWWTY3W/g9PBxXNxfWhfbrm4Le root@client
   '';
 
···
       {
         services.gitolite = {
           enable = true;
-          adminPubkey = builtins.readFile adminPublicKey;
+          adminPubkey = adminPublicKey;
         };
         services.openssh.enable = true;
       };