gnu: coreutils: Update build phase.

* gnu/packages/base.scm (coreutils)[arguments]: Use 'modify-phases'
syntax. Substitute with 'which sh'.

gnu/packages/base.scm

@@ -320,21 +320,18 @@ (define-public coreutils
    (outputs '("out" "debug"))
    (arguments
     `(#:parallel-build? #f            ; help2man may be called too early
-      #:phases (alist-cons-before
-                'build 'patch-shell-references
-                (lambda* (#:key inputs #:allow-other-keys)
-                  (let ((bash (assoc-ref inputs "bash")))
+      #:phases (modify-phases %standard-phases
+                 (add-before 'build 'patch-shell-references
+                   (lambda _
                      ;; 'split' uses either $SHELL or /bin/sh.  Set $SHELL so
                      ;; that tests pass, since /bin/sh isn't in the chroot.
                      (setenv "SHELL" (which "sh"))
 
                      (substitute* (find-files "gnulib-tests" "\\.c$")
-                      (("/bin/sh")
-                       (format #f "~a/bin/sh" bash)))
+                       (("/bin/sh") (which "sh")))
                      (substitute* (find-files "tests" "\\.sh$")
-                      (("#!/bin/sh")
-                       (format #f "#!~a/bin/sh" bash)))))
-                %standard-phases)))
+                       (("#!/bin/sh") (which "sh")))
+                     #t)))))
    (synopsis "Core GNU utilities (file, text, shell)")
    (description
     "GNU Coreutils includes all of the basic command-line tools that are