Add `emacs-next' for building latest Emacs from git.
* gnu/packages/emacs.scm (emacs-next): New variable.
(emacs): make the autoload deletion snippet not fail when eshell/esh-groups.el
does not exist. This enables reuse of the entire snippet field of `emacs' for
`emacs-next'.
* gnu/packages/patches/emacs27-exec-path.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add the above patch file to it.
Signed-off-by: Ludovic Courtès <ludo@gnu.org>
