…ODE_AUTH_TOKEN

This change addresses issue actions#1440 where NPM OIDC authentication was broken
because the action was exporting a fake NODE_AUTH_TOKEN value by default.

NPM OIDC requires NODE_AUTH_TOKEN to either be unset or empty for proper
authentication. The fix only exports NODE_AUTH_TOKEN if it was explicitly
set by the user, allowing OIDC to work while maintaining backward compatibility
for users who explicitly provide tokens.

BREAKING CHANGE: Users who rely on the fake default token should now explicitly
provide NODE_AUTH_TOKEN in their workflows or use OIDC authentication.

Fixes actions#1440
Related: actions#1440