nixos/github-runner: use hostname as default for option name

Until now, the runner registration did not set the `--name` argument if
the configuration option was `null`, the default for the option.
According to GitHub's documentation, this instructs the registration
script to use the machine's hostname.

This commit causes the registration to always pass the `--name` argument
to the runner configuration script. The option now defaults to
`networking.hostName` which should be always set on NixOS.

This change becomes necessary as the systemd service name includes the
name of the runner since fcfa809 and, hence, expects it to be set. Thus,
an unset `name` option leads to an error.

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

@@ -46,14 +46,14 @@ in
     };
 
     name = mkOption {
-      type = types.nullOr types.str;
+      type = types.strMatching "^$|^[[:alnum:]]([[:alnum:]_-]{0,61}[[:alnum:]])?$";
       description = ''
         Name of the runner to configure. Defaults to the hostname.
 
         Changing this option triggers a new runner registration.
       '';
       example = "nixos";
-      default = null;
+      default = config.networking.hostName;
     };
 
     runnerGroup = mkOption {
@@ -200,7 +200,7 @@ in
                   --url '${cfg.url}' \
                   --token "$token" \
                   --labels '${concatStringsSep "," cfg.extraLabels}' \
-                  ${optionalString (cfg.name != null) "--name '${cfg.name}'"} \
+                  --name '${cfg.name}' \
                   ${optionalString cfg.replace "--replace"} \
                   ${optionalString (cfg.runnerGroup != null) "--runnergroup '${cfg.runnerGroup}'"}