Variable DEFAULT_NX_CLOUD_ACCESS_TOKEN_EXPRESSIONConst

Default GitHub Actions expression for the NX_CLOUD_ACCESS_TOKEN env var.

Prefers a write-scope token (NX_CLOUD_AUTH_TOKEN_WRITE) on main-branch pushes so CI warms the distributed cache; falls back to the read-only token (NX_CLOUD_ACCESS_TOKEN) on PRs, forks, and when the write secret is absent. Missing secrets evaluate to empty — safe.