gnu: font-go: Use 'font-build-system'.

* gnu/packages/fonts.scm (font-go): Switch to font-build-system.
Re-indent.

Signed-off-by: Danny Milosavljevic <dannym@scratchpost.org>

gnu/packages/fonts.scm

@@ -1237,20 +1237,14 @@ programming.  Iosevka is completely generated from its source code.")
                 (sha256
                  (base32
                   "1aq6mnjayks55gd9ahavk6jfydlq5lm4xm0xk4pd5sqa74p5p74d"))))
-     (build-system trivial-build-system)
+      (build-system font-build-system)
       (arguments
-      `(#:modules ((guix build utils))
-        #:builder (begin
-                    (use-modules (guix build utils))
-                    (let ((font-dir (string-append %output
-                                                   "/share/fonts/truetype"))
-                          (source (assoc-ref %build-inputs "source")))
-                      (mkdir-p font-dir)
-                      (with-directory-excursion
-                       (string-append source "/font/gofont/ttfs")
-                       (for-each (lambda (ttf)
-                                   (install-file ttf font-dir))
-                                 (find-files "." "\\.ttf$")))))))
+       `(#:phases
+         (modify-phases %standard-phases
+           (add-before 'install 'chdir
+             (lambda _
+               (chdir "font/gofont/ttfs")
+               #t)))))
       (home-page "https://blog.golang.org/go-fonts")
       (synopsis "The Go font family")
       (description