Prevent eshell-path-env getting out-of-sync with $PATH

[dkellner]

When changing .envrc in a running Eshell session, changes to $PATH don't have any effect:

Welcome to the Emacs shell

$ mkdir foo && cd foo
$ echo "#!/bin/sh" > bar.sh
$ echo "echo 'works!'" >> bar.sh
$ chmod +x bar.sh
$ ./bar.sh
works!

$ echo "PATH_add ." > .envrc
$ direnv allow
$ direnv-update-directory-environment
direnv: ~PATH (~/dev/emacs-direnv/foo)
$ echo $PATH
/home/dkellner/dev/emacs-direnv/foo:...

$ bar.sh
bar.sh: command not found
$ sh -c "bar.sh"
works!

After updating eshell-path-env to the current value of $PATH, the behaviour is as expected:

$ (setq eshell-path-env (getenv "PATH"))
/home/dkellner/dev/emacs-direnv/foo:...
$ bar.sh
works!

[wbolster]

i don't use eshell myself, but this looks good

[wbolster]

instead of using eq to check major-mode, i think it's better to use derived-mode-p instead

[wbolster]

i've updated the pr. looks good now.