Sven Brosi what means "If a npmrc file is created inside a folder" ?
The dropdown in build step or build environment option shows only files of kind "Npm config file" present in "Config File Management" through the config plugin.
If you want nodejs use a custom .npmrc placed somewhere than add npm_config_userconfig as environment variable that point to your file. Consider that npm lookup some default places. Have a look here https://docs.npmjs.com/files/npmrc#files.
For example if you put the .npmrc in the root of the job workspace is it used by default and have precedence on other files