Fix lifecycle to pass `Infinity` for config env value

The default value of depth config is Infinity and lifecycle.js was writing its value with `JSON.stringify`. Since `JSON.stringify(Infinity)` is `"null"`, `npm_config_depth` env was set to `"null"`. This led depth value to be read 0 within child process.
Also this causes tap(prune) test fail with https://github.com/npm/read-installed/pull/13 .