build: emacs-utils: Fix `emacs-substitute-variables'.

* guix/build/emacs-utils.scm (emacs-substitute-variables): Fix the regexp
matching variable name.

With end-of-word regexp ("\>"), the previous regexp may match the prefix of
a variable only, effectively deleting the rest of its name.

For example,

  (emacs-substitute-variables "file.el"
    ("foo" ...))

could match (defvar foo-bar ...) and replace it with (defvar foo ...).

guix/build/emacs-utils.scm

@@ -220,7 +220,7 @@ useful to avoid double quotes being added when the replacement is provided as
 a string."
     ((_ file (variable replacement modifier ...) ...)
      (emacs-substitute-sexps file
-       ((string-append "(def[a-z]+[[:space:]\n]+" variable "\\>")
+       ((string-append "(def[a-z]+[[:space:]\n]+" variable "\\_>")
         replacement
         modifier ...)
        ...))))