nixos/github-runner: create a parent for systemd dirs

Adds a parent directory "github-runner/" to all of the systemd lifecycle
directories StateDirectory=, RuntimeDirectory= and LogDirectory=.

Doing this has two motivations:

1. Something like this would required if we want to support multiple
   runners configurations. Please note that this is already possible
   using NixOS containers.
2. Having an additional parent directory makes it easier to remap
   any of the directories. Without a parent, systemd is going to
   complain if, for example, the given StateDirectory= is a symlink.

nixos/modules/services/continuous-integration/github-runner.nix

@@ -2,8 +2,10 @@
 with lib;
 let
   cfg = config.services.github-runner;
-  name = "github-runner";
-  runnerRoot = "/run/${name}"; # RuntimeDirectory=
+  svcName = "github-runner";
+  systemdDir = "${svcName}/${cfg.name}";
+  systemdUser = "${svcName}-${cfg.user}";
+  runnerRoot = "/run/${systemdDir}"; # RuntimeDirectory=
 in
 {
   options.services.github-runner = {
@@ -101,7 +103,7 @@ in
       ''
     ];
 
-    systemd.services.${name} = {
+    systemd.services.${svcName} = {
       description = "GitHub Actions runner";
 
       wantedBy = [ "multi-user.target" ];
@@ -139,7 +141,7 @@ in
           let
             currentConfigPath = "$STATE_DIRECTORY/.nixos-current-config.json";
             runnerRegistrationConfig = getAttrs [ "name" "tokenFile" "url" "runnerGroup" "extraLabels" ] cfg;
-            newConfigPath = builtins.toFile "${name}-config.json" (builtins.toJSON runnerRegistrationConfig);
+            newConfigPath = builtins.toFile "${svcName}-config.json" (builtins.toJSON runnerRegistrationConfig);
             currentConfigTokenFilename = ".current-token";
             newConfigTokenFilename = ".new-token";
             runnerCredFiles = [
@@ -233,20 +235,18 @@ in
           ];
 
         # Contains _diag
-        LogsDirectory = name;
+        LogsDirectory = [ systemdDir ];
         # Default RUNNER_ROOT which contains ephemeral Runner data
-        RuntimeDirectory = name;
+        RuntimeDirectory = [ systemdDir ];
         # Home of persistent runner data, e.g., credentials
-        StateDirectory = name;
+        StateDirectory = [ systemdDir ];
         StateDirectoryMode = "0700";
         WorkingDirectory = runnerRoot;
 
-        # By default, use a dynamically allocated user with a name and group
-        # equal to the service unit name (which is the systemd default for
-        # DynamicUser=true; just set explicitly for the sake of clarity).
+        # By default, use a dynamically allocated user
         DynamicUser = true;
-        User = name;
-        Group = name;
+        User = systemdUser;
+        Group = User;
 
         KillMode = "process";
         KillSignal = "SIGTERM";