use BTreeSet for ruleset rules

[marcoieni]

This PR fixes the empty sync-team diff we see in PRs

[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

[Kobzol]

I was thinking about this when examining the implementation; I hoped that GitHub keeps the order. Seems not, so keeping it sorted it is.

Even better would be to actually represent the rules as fields, rather than in a container, because we use at most one rule per type anyway. But that's a more complex change.

[marcoieni]

It failed with the following error:

    📝 Editing repo 'rust-lang/bors':
      Rulesets:
          Updating 'Ruleset for main'
            Bypass Actors: Some([]) => None
    📝 Editing repo 'rust-lang/thorin':
      Branch Protections:
          main
            Dismiss Stale Reviews: false
            Is admin enforced: true
            Required Approving Review Count: 1
            Requires PR: true
            Required Checks: []
            Allowances: []
    📝 Editing repo 'rust-lang/triagebot':
      Branch Protections:
          master
            Dismiss Stale Reviews: false
            Is admin enforced: true
            Required Approving Review Count: 1
            Requires PR: true
            Required Checks: ["ci"]
            Allowances: []
    
Error: rust_team] failed: graphql error: Name already protected: main

[marcoieni]

I'll try again to merge this. Maybe some flakiness from github..