build-system: emacs: Do not rely on input labels in the 'build' phase.

This commit repairs the Emacs build system for packages using the new
label-less "guix style".  Tested with emacs-libgit.

* guix/build/emacs-build-system.scm (build): Use search-input-file to locate
emacs.

guix/build/emacs-build-system.scm

@@ -110,7 +110,7 @@ environment variable\n" source-directory))
 
 (define* (build #:key outputs inputs #:allow-other-keys)
   "Compile .el files."
-  (let* ((emacs (string-append (assoc-ref inputs "emacs") "/bin/emacs"))
+  (let* ((emacs (search-input-file inputs "/bin/emacs"))
          (out (assoc-ref outputs "out")))
     (setenv "SHELL" "sh")
     (parameterize ((%emacs emacs))