pkgs = import sources.nixpkgs {
    overlays = [
      (super: self: {
        nodejs = self.nodejs-15_x;
      })
    ];

❯ nix-build .travis.nix
trace: Warning: `stdenv.lib` is deprecated and will be removed in the next release. Please use `lib` instead. For more information see https://github.com/NixOS/nixpkgs/issues/108938
these derivations will be built:
  /nix/store/gj5xjdva2fw6fwsq08fjpbknmi06ligl-simple-test-1.0.0-node-modules.drv
  /nix/store/mi1zc0jyjmz7d7jflmkf56k2q457nwqj-simple-test-1.0.0.drv
building '/nix/store/gj5xjdva2fw6fwsq08fjpbknmi06ligl-simple-test-1.0.0-node-modules.drv'...
building npm cache
building node_modules
7.7.6
npm ERR! code ENOTCACHED0m reify:loadBundles Compl
npm ERR! request to https://registry.npmjs.org/yargs-parser/-/yargs-parser-13.1.1.tgz failed: cache mode is 'only-if-cached' but no cached response available.

npm ERR! A complete log of this run can be found in:
npm ERR!     /build/npm-cache/_logs/2021-05-10T09_06_14_404Z-debug.log

builder for '/nix/store/gj5xjdva2fw6fwsq08fjpbknmi06ligl-simple-test-1.0.0-node-modules.drv' failed with exit code 1
cannot build derivation '/nix/store/mi1zc0jyjmz7d7jflmkf56k2q457nwqj-simple-test-1.0.0.drv': 1 dependencies couldn't be built
error: build of '/nix/store/mi1zc0jyjmz7d7jflmkf56k2q457nwqj-simple-test-1.0.0.drv' failed