direnv-mode ignores custom exec-path entries

[acid-bong]

Expected behaviour: when I enter a direnv-managed directory, the new paths are expected to be prepended to exec-path.

Actual behaviour: exec-path is fully redefined.

Nix-related cases, where it'd better to modify exec-path instead of redefining it:

• nix's script, that wraps Emacs with packages in a single closure, adds (add-to-list 'exec-path "$out/bin") to site-start.el, thus prepending the wrapped Emacs closure to its path
• since I manage my Emacs with nix, I want to add some tools to Emacs' path, like LSPs, formatters, linters and so on

Possible culprit: https://github.com/wbolster/emacs-direnv/blob/main/direnv.el#L270-L271