gnu: emacs-buttercup: Improve package style.

* gnu/packages/emacs-xyz.scm (emacs-buttercup)[arguments]: Use
G-expressions. Remove trailing #T from phases.

gnu/packages/emacs-xyz.scm

@@ -24012,15 +24012,15 @@ targets the Emacs based IDEs (CIDER, ESS, Geiser, Robe, SLIME etc.)")
          "1zr9lczdr9dfh99n079d5advxm9p32a8f984dx14f3jn5mx750b1"))))
     (build-system emacs-build-system)
     (arguments
-     `(#:tests? #t
-       #:test-command '("make" "test")
-       #:phases
-       (modify-phases %standard-phases
-         (add-after 'install 'install-bin
-           (lambda* (#:key outputs #:allow-other-keys)
-             (install-file "bin/buttercup"
-                           (string-append (assoc-ref outputs "out") "/bin"))
-             #t)))))
+     (list
+      #:tests? #t
+      #:test-command #~(list "make" "test")
+      #:phases
+      #~(modify-phases %standard-phases
+          (add-after 'install 'install-bin
+            (lambda _
+              (install-file "bin/buttercup"
+                            (string-append #$output "/bin")))))))
     (home-page "https://github.com/jorgenschaefer/emacs-buttercup")
     (synopsis "Behavior driven emacs lisp testing framework")
     (description "Buttercup is a behavior-driven development framework for