feat: Support for setting runner environment variables

[kichik]

const myBuilder = FargateRunnerProvider.imageBuilder(this, 'image builder');
myBuilder.addComponent(
  RunnerImageComponent.environmentVariables({
    HELLO: 'world',
    FOO: 'bar',
  })
);

const myProvider = new FargateRunnerProvider(this, 'fargate runner', {
   labels: ['fargate-with-env-vars'],
   imageBuilder: myBuilder,
});

new GitHubRunners(this, 'runners', {
   providers: [myProvider],
});

Resolves #509