Read ARM leases from public main branch (#45699)

ARM Modeling Review now checks out .github/arm-leases from the main branch of the public Azure/azure-rest-api-specs repo into a separate folder, and reads leases from there instead of from the PR's own branch. A lease merged to public main is honored by PRs in every repo and branch, so leases no longer need to be synced into RPSaaSMaster/RPSaaSDev.