Fixes 500 error on dashboard when using MSSQL (go-gitea#2504)

MSSQL rejects the query:
  SELECT "repository"."id" FROM "repository"
  INNER JOIN "team_repo" ON "team_repo".repo_id="repository".id AND
  "repository".is_mirror=1
  WHERE (
    ("repository".owner_id=2 AND "repository".is_private=0)
        OR team_repo.team_id IN (1)
        )
        GROUP BY "repository".id ORDER BY updated_unix DESC

when the order by term (updated_unix) is not included in the group by
term.

Author: bradder555

models/org.go

@@ -671,7 +671,7 @@ func (env *accessibleReposEnv) MirrorRepoIDs() ([]int64, error) {
 		Table("repository").
 		Join("INNER", "team_repo", "`team_repo`.repo_id=`repository`.id AND `repository`.is_mirror=?", true).
 		Where(env.cond()).
-		GroupBy("`repository`.id").
+		GroupBy("`repository`.id, `repository`.updated_unix").
 		OrderBy("updated_unix DESC").
 		Cols("`repository`.id").
 		Find(&repoIDs)