gnu: grub-minimal: Fix cross-compilation.

Fixes a regression introduced in
34a6f12351 that would break
cross-compilation of 'grub-minimal' because it would enable Freetype
support as a side effect but the 'set-freetype-variables' phase was not
inherited in 'grub-minimal'.

* gnu/packages/bootloaders.scm (grub-minimal)[arguments]: Use
'substitute-keyword-arguments' so that the 'set-freetype-variables'
phase is inherited.

gnu/packages/bootloaders.scm

@@ -258,9 +258,14 @@ (define-public grub-minimal
      (fold alist-delete (package-native-inputs grub)
            '("help2man" "texinfo" "parted" "qemu" "xorriso")))
     (arguments
-     `(#:configure-flags (list "PYTHON=true")
-       #:phases (modify-phases %standard-phases
-                  (add-after 'unpack 'patch-stuff
+     (substitute-keyword-arguments (package-arguments grub)
+       ((#:configure-flags _ ''())
+        '(list "PYTHON=true"))
+       ((#:tests? _ #t)
+        #f)
+       ((#:phases phases '%standard-phases)
+        `(modify-phases ,phases
+           (replace 'patch-stuff
              (lambda* (#:key native-inputs inputs #:allow-other-keys)
                (substitute* "grub-core/Makefile.in"
                  (("/bin/sh") (which "sh")))
@@ -271,8 +276,7 @@ (define-public grub-minimal
                           "unifont.bdf.gz")
                (system* "gunzip" "unifont.bdf.gz")
 
-                     #t)))
-       #:tests? #f))))
+               #t))))))))
 
 (define-public grub-efi
   (package