fix: parser doesn't validate against options parameter if the value is provided through an env var

[rafaelmotaalves]

Hey, I found this issue that when I try to pass a flag as an environment variable it is not validated if this value is on the options field.

An example would be

export default class MyCommand extends Command {
  static flags = {
    foo: Flags.string({
      env: 'FOO',
      options: ['valid1', 'valid2']
    })
  }
}

My understanding here is that if I execute the command with the FOO environment variable as invalid the parsing should fail and an error message should be displayed, this is not happening.

I added a couple of tests that verify this behavior and fixed it on this PR.

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @rafaelmotaalves to sign the Salesforce.com Contributor License Agreement.