Shave the build-number yak

[hmemcpy]

Seems that github actions deprecated set-env based actions. Need to upgrade to whatever is recommended.
P.S. the author of the build-number action suggest:

GitHub has just introduced new environment variables, GITHUB_RUN_ID and GITHUB_RUN_NUMBER which are unique numbers for each workflow run, so you're probably better off using those than this GitHub action. 🙂 See https://help.github.com/en/actions/configuring-and-managing-workflows/using-environment-variables#default-environment-variables