CI: Replace set-env in Cygwin

[ax3l]

Use the GitHub Action env: keyword instead of the deprecated set-env command:

• 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#adding-a-system-path
  • https://docs.github.com/en/free-pro-team@latest/actions/reference/workflow-commands-for-github-actions#environment-files
• https://github.community/t/migration-from-deprecated-add-path-on-windows/136265

[ax3l]

MSYS2 migration needs #821