set-env and add-path are deprecated in Github Actions

[reikdas]

set-env and add-path are deprecated in Github Actions. I think your script still uses them because I can see a bunch of warnings in the last Actions run in this repository - https://github.com/GiovanniBussi/macports-ci/actions/runs/299832976 and I also get a bunch of warnings downstream in my repository - https://github.com/reikdas/cling/actions/runs/309226905. Could these please be fixed? Thanks!