gnu: emacs-haskell-snippets: Use correct directory for snippets.

This package instructs Yasnippet to look for snippets relative to its own
path.  However, it installs the snippets directly to site-lisp, which as of
79cfe30f3e is one directory above that.  Use the
elpa-directory procedure introduced in that commit to rectify this mistake.

* gnu/packages/emacs-xyz.scm (emacs-haskell-snippets)[#:phases]
<install-snippets>: Use elpa-directory.

Fixes: Wrong location for haskell snippets <https:issues.guix.gnu.org/66562>

gnu/packages/emacs-xyz.scm

@@ -35632,10 +35632,11 @@ (define-public emacs-haskell-snippets
         #:phases
         #~(modify-phases %standard-phases
             (add-after 'install 'install-snippets
-              (lambda _
+              (lambda* (#:key outputs #:allow-other-keys)
                 (let ((snippets
                        (string-append
-                        #$output "/share/emacs/site-lisp/snippets/haskell-mode")))
+                        (elpa-directory (assoc-ref outputs "out"))
+                        "/snippets/haskell-mode")))
                   (mkdir-p snippets)
                   (copy-recursively "snippets/haskell-mode" snippets)))))))
       (propagated-inputs