fix IS_NIGHTLY env uses

[poszu]

Closes #200

The existing uses of IS_NIGHTLY env were written in a way that works with the variable either set or not existing at all. The PR #170 enabled nightly builds by uncommenting:

env:
  IS_NIGHTLY: ${{ github.event_name == 'schedule' || github.event_name == 'workflow_dispatch' }}

This version, however results in the IS_NIGHTLY variable always present with a value true or false. This causes the workflow to be in "nightly mode" always (see https://github.com/athenavm/athena/actions/runs/11572825493/job/32213626274).

This PR fixes uses of the variable in the release workflow to work with IS_NIGHTLY=false|true.

Non-nightly run (on v* tag, nightly steps are skipped): https://github.com/athenavm/athena/actions/runs/11575072142/job/32220789839
Nightly run (on workflow_dispatch): https://github.com/athenavm/athena/actions/runs/11575009673/job/32220734895