fix(cli/flags.rs): Prevent providing --allow-env flag twice

[niceb5y]

  if self.allow_env {
      args.push("--allow-env".to_string());
    }

was duplicated in cli/flag.rs

Duplication causes the problem when deno install --allow-env

error: The argument '--allow-env' was provided more than once, but cannot be used multiple times

USAGE:
    deno run --allow-env --allow-net=<allow-net> --allow-read=<allow-read>

For more information try --help

[claassistantio]

All committers have signed the CLA.

[ry]

Thanks! Just one comment...

[ry]

Add a comment about how allow_env is implied. Or filter it during install.

[niceb5y]

‘allow_env’ already processed L142-L144

Or am i miss some intentions that it is processed twice?

[ry]

oops! no- thank you!

[ry]

LGTM