Nixpacks silently falls back to its own default Node version when a
repo doesn't match what it expects, instead of failing the build —
confirmed in practice as a green build that crashed at runtime on a
missing global. detectNodeVersion() reads the repo's own .nvmrc or
package.json engines.node and passes it through as
NIXPACKS_NODE_VERSION so the build actually uses what the repo
declares.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
