bug#44726: 28.0.50; [feature/native-comp] Provide a mechanism to populate comp-eln-load-path from the environment

[Tad]

Emacs populates `load-path' at startup from `$EMACSLOADPATH' in the
process environment. It would be useful to have an
`$EMACSNATIVELOADPATH' or equivalent to populate `comp-eln-load-path'.

I have implemented a workaround in NixOS with some code in
`site-start.el', but it would probably be useful in other contexts as
well.

    ;; Append paths set via `EMACSNATIVELOADPATH', an environment
    ;; variable made up for this purpose.
    (when-let ((path-env (getenv "EMACSNATIVELOADPATH"))
               (paths-from-env (split-string path-env))
               (paths (seq-filter (lambda (path) (not (string= "" path)))
                                  paths-from-env)))
      (setq comp-eln-load-path
            (append paths comp-eln-load-path)))

Thanks,
Tad