Use the new set output instead of the deprecated one

[nyurik]

Per https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/ - output will soon be deprecated, and should be trivial to change (although I don't have any way to validate it).

P.S. You might be interested in my github action code to install Postgis on Windows -- ideally this could be integrated into your action (together with something for mac & linux) - to make things far simpler.

Thanks for the code!