JUPYTER_CONFIG_PATH does not work.

**Describe the bug**

`JUPYTER_CONFIG_PATH` does not work.

**To Reproduce**

```
mkdir custom
touch custom/custom.js
echo "console.log('test')" > custom/custom.js
JUPYTER_CONFIG_PATH=$(pwd) jupyter notebook
```

**Expected behavior**

The file should be loaded; `test` should be printed in the console.

**Desktop (please complete the following information):**

- OS: macOS 12.3.1
- Browser Safari 15.4, Chromium 105.0.5175.0 (Developer Build)

Reproduce the issue with the listed mkdir, custom/custom.js, and JUPYTER_CONFIG_PATH commands, then trace how Jupyter Notebook discovers and loads configuration files. Confirm the fix by rerunning the command and verifying that `test` is printed in the console.

Written by the indexing model from the issue text.