[Bug]: RUNFILES_DIR should be in the environment of a JS binary

### What happened?

when spawning a subprocess, like a `go_binary`, it will expect this variable to be set.

https://docs.google.com/document/d/e/2PACX-1vSDIrFnFvEYhKsCMdGdD40wZRBX3m3aZ5HhVj4CtHPmiXKDCxioTUbYsDydjKtFDAzER5eg7OjJWs3V/pub is an old doc for this. @fmeum has done a lot of runfiles work last year.

Currently we export `RUNFILES` instead, is it just misnamed?
https://github.com/aspect-build/rules_js/blob/main/js/private/js_binary.sh.tpl#L134
and it has a TODO(2.0) to delete it.

### Version

HEAD

### How to reproduce

_No response_

### Any other information?

_No response_