Add the environment variable for using Shepherd

[ngmy]

I want to be able to specify whether or not to use Shepherd by the environment variable.

I use a Composer script to run lint in both my local and CI environments (Github Actions):

"scripts":
    "lint": [
        "@phpcs",
        "@psalm"
    ],
    "phpcs": "@php tools/phpcs ...",
    "psalm": "@php tools/psalm ...",
}

I would be happy if it could be specified by the environment variable, so that I don't have to write scripts that only differ in the presence or absence of the --shepherd option:

- name: Run lint
  env:
      PSALM_SHEPHERD: 1
      run: composer lint

or

- name: Run lint
  env:
      PSALM_SHEPHERD: 1
      PSALM_SHEPHERD_HOST: host
      run: composer lint

[weirdan]

Thanks!