This would be the answer to the question.

import { DataSource } from 'typeorm';

export function Transactional() {
  return function (target: any, propertyName: string, descriptor: TypedPropertyDescriptor<any>) {
    const method = descriptor.value;

    descriptor.value = async function (...args: any[]) {
      const dataSource: DataSource = this.dataSource;

      const queryRunner = dataSource.createQueryRunner();
      await queryRunner.startTransaction();

      const createQueryRunner = dataSource.createQueryRunner;
      const release = queryRunner.release;

      dataSource.createQueryRunner = () => queryRunner;
      queryRunner.release = () => Promise.resolve();

      try {
        const result = await method.apply(this, args);
        await queryRunner.commitTransaction();
        return result;
      } catch (err) {
        await queryRunner.rollbackTransaction();
        throw err;
      } finally {
        dataSource.createQueryRunner = createQueryRunner;
        queryRunner.release = release;
        await queryRunner.release();
      }
    };
  };
}

But I also had to add private dataSource: DataSource parameter to the constructor of BrandsService, because it is used in the decorator.

import { ConflictException, Inject, Injectable, NotFoundException } from '@nestjs/common';
import { DataSource, Repository } from 'typeorm';
import { Brand } from './brand.entity';
import { BRANDS_REPOSITORY } from './brands.repository';
import { USERS_REPOSITORY } from '../users/users.repository';
import { User } from '../users/user.entity';
import { DATA_SOURCE } from '../database/database.providers';
import { Transactional } from '../../decorators/transactional.decorator';

@Injectable()
export class BrandsService {
  constructor(
    @Inject(BRANDS_REPOSITORY) private brandsRepository: Repository<Brand>,
    @Inject(USERS_REPOSITORY) private usersRepository: Repository<User>,
    @Inject(DATA_SOURCE) private dataSource: DataSource, // <<==
  ) {}

  @Transactional()
  async create(user: User, name: string) {
    let brand = await this.brandsRepository.create({ name });
    brand = await this.brandsRepository.save(brand);

    user.permittedBrands.push(brand);
    await this.usersRepository.save(user);

    throw new Error('Something went wrong');

    return brand;
  }
}