Actions: set-env is disabled, breaking Actions workflows

[derrickstolee]

See .github/workflows/continuous-integration.yml for an example:

      run: echo "::set-env name=BUILD_PLATFORM::${{ runner.os }}"

We need to update the YML so we can get builds working again.

[dscho]

PR: #466

[derrickstolee]

Resolved by #466.