relax github team members validation

[marcoieni]

Sometimes GitHub usernames change and our CI fails.
With this PR, the CI fails only if the user is a member of a team (not alumni).

Manual test 1

1. Change the username of a member (eg https://github.com/rust-lang/team/blob/main/people/maurer.toml )
   • Change the file name
   • Change the github field in the file
   • Change their username in any members section they appear.
2. Run GITHUB_TOKEN=$(gh auth token) cargo run -- check

Output:

[ERROR rust_team::validate] validation error: GitHub user `maurer1` changed username to `maurer`
[ERROR rust_team] 1 validation errors found

Manual test 2

1. Change a username of an alumni ( eg https://github.com/rust-lang/team/blob/main/people/J-ZhengLi.toml )
   • Change the file name
   • Change the github field in the file
   • Change their username in any alumni section they appear.
2. Run the previous command

The output doesn't report errors

[github-actions]

Dry-run check results

[WARN  rust_team::sync] sync-team is running in dry mode, no changes will be applied.
[INFO  rust_team::sync] synchronizing crates-io
[INFO  rust_team::sync] synchronizing github

[marcoieni]

I preferred to include these teams in the validation because imo it is important that the usernames are up-to-date.
Eg https://github.com/rust-lang/team/blob/main/teams/android.toml