feat: add adminUsername option back to NixOS module

flake.nix

@@ -95,6 +95,12 @@
               description = "Bcrypt hash of the admin bootstrap password.";
             };
 
+            adminUsername = lib.mkOption {
+              type = lib.types.str;
+              default = "admin";
+              description = "Username of the admin user created at bootstrap.";
+            };
+
             sqlitePath = lib.mkOption {
               type = lib.types.path;
               default = "/var/lib/st-pierre/st-pierre.sqlite";
@@ -155,6 +161,9 @@
                 }
                 // lib.optionalAttrs (cfg.adminPasswordHash != null) {
                   ADMIN_PASSWORD_HASH = cfg.adminPasswordHash;
+                }
+                // {
+                  ADMIN_USERNAME = cfg.adminUsername;
                 };
             };
           };