nixos/buildkite: drop user option (#78160)

* nixos/buildkite: drop user option

This reverts 8c6b1c3eaa.

Turns out, buildkite-agent has logic to write .ssh/known_hosts files and
only really works when $HOME and the user homedir are in sync.

On top of that, we provision ssh keys in /var/lib/buildkite-agent, which
doesn't work if that other users' homedir points elsewhere (we can cheat
by setting $HOME, but then getent and $HOME provide conflicting
results).

So after all, it's better to only run the system-wide buildkite agent as
the "buildkite-agent" user only - if one wants to run buildkite as
different users, systemd user services might be a better fit.

* nixosTests.buildkite-agent: add node with separate user and no ssh key

nixos/modules/services/continuous-integration/buildkite-agent.nix

@@ -29,8 +29,6 @@ let
     ${concatStringsSep "\n" (mapAttrsToList mkHookEntry (filterAttrs (n: v: v != null) cfg.hooks))}
   '';
 
-  defaultUser = "buildkite-agent";
-
 in
 
 {
@@ -58,15 +56,6 @@ in
         type = types.listOf types.package;
       };
 
-      user = mkOption {
-        type = types.str;
-        default = defaultUser;
-        description = ''
-          Set this option when you want to run the buildkite agent as something else
-          than the default user "buildkite-agent".
-        '';
-      };
-
       tokenPath = mkOption {
         type = types.path;
         description = ''
@@ -197,7 +186,7 @@ in
   };
 
   config = mkIf config.services.buildkite-agent.enable {
-    users.users.buildkite-agent = mkIf (cfg.user == defaultUser) {
+    users.users.buildkite-agent = {
       name = "buildkite-agent";
       home = cfg.dataDir;
       createHome = true;
@@ -242,7 +231,7 @@ in
 
         serviceConfig =
           { ExecStart = "${cfg.package}/bin/buildkite-agent start --config /var/lib/buildkite-agent/buildkite-agent.cfg";
-            User = cfg.user;
+            User = "buildkite-agent";
             RestartSec = 5;
             Restart = "on-failure";
             TimeoutSec = 10;