feat: add environment variable support

[bahmutov]

• closes Add environment variable #11

// run tests with 
// CYPRESS_ENVIRONMENT=staging npx cypress run
import {onlyOn} from '@cypress/skip-test'
const stubServer = () => {
  cy.server()
  cy.route('/api/me', 'fx:me.json')
  cy.route('/api/permissions', 'fx:permissions.json')
  // Lots of fixtures ...
}
it('works', () => {
  onlyOn('staging', stubServer)
  ...
})

[bahmutov]

🎉 This PR is included in version 2.3.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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