Build-System: Replace obsolete set-env in github actions

[jahnf]

set-env in github actions is deprecated and is going to be disabled soon, replace with them with the new syntax:

See:

• https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/
• https://docs.github.com/en/free-pro-team@latest/actions/reference/workflow-commands-for-github-actions#environment-files