emacs_requirement: set $EMACS

The Emacs shell sets $EMACS to "t" for detection purposes, but it causes
builds to fail when they attempt to call Emacs using the variable.

Fixes Homebrew/homebrew-emacs#30.

Closes Homebrew/homebrew#45495.

Signed-off-by: Alex Dunn <dunn.alex@gmail.com>

Library/Homebrew/requirements/emacs_requirement.rb

@@ -16,6 +16,7 @@ class EmacsRequirement < Requirement
 
   env do
     ENV.prepend_path "PATH", which("emacs").dirname
+    ENV["EMACS"] = "emacs"
   end
 
   def message