Don't return duplicated users who can create org repo (#22560) (#22562)

- Backport of #22560
- Currently the function `GetUsersWhoCanCreateOrgRepo` uses a query that
is able to have duplicated users in the result, this is can happen under
the condition that a user is in team that either is the owner team or
has permission to create organization repositories.
- Add test code to simulate the above condition for user 3,
[`TestGetUsersWhoCanCreateOrgRepo`](https://github.com/go-gitea/gitea/blob/a1fcb1cfb84fd6b36c8fe9fd56588119fa4377bc/models/organization/org_test.go#L435)
is the test function that tests for this.
  - The fix is quite trivial, use a map as a set to get distinct orgs.

models/fixtures/user.yml

@@ -104,7 +104,7 @@
   num_following: 0
   num_stars: 0
   num_repos: 3
-  num_teams: 4
+  num_teams: 5
   num_members: 3
   visibility: 0
   repo_admin_change_team_access: false