Sets FORCE_JAVASCRIPT_ACTIONS_TO_NODE24=true at the workflow env level
so all JavaScript-based actions (actions/checkout, actions/setup-dotnet,
actions/upload-artifact, actions/download-artifact, softprops/action-gh-release)
run under Node 24 instead of the deprecated Node 20 runtime.

GitHub is auto-migrating JavaScript actions to Node 24 as the default on
June 2, 2026 and removing the Node 20 runner entirely on September 16,
2026. Opting in now:

- Removes the deprecation warnings from the most recent release runs.
- Surfaces any Node 24 compatibility issues while there's still time to
  downgrade via ACTIONS_ALLOW_USE_UNSECURE_NODE_VERSION=true.
- No action version bumps needed — the flag reuses the existing v4/v2
  pins, which continue working under Node 24.

The inline comment documents the Jun 2 -> Sep 16 fallback escape hatch
window explicitly (ACTIONS_ALLOW_USE_UNSECURE_NODE_VERSION only works
while the Node 20 runtime is still installed on the runner; after
Sep 16, 2026 there is no runtime to fall back to). The flag itself
becomes redundant on June 2 and should be removed then.