gnu: emacs: Patch references to /bin/sh.

When attempting to run ert-runner in the build side environment where /bin/sh
doesn't exist as part of the emacs-elpy test suite, Emacs was throwing the
error "Searching for program: No such file or directory, /bin/sh".  This is
because the default shell when SHELL is not set was "/bin/sh", as can be seen
in callproc.c:

  sh = getenv ("SHELL");
  Vshell_file_name = build_string (sh ? sh : "/bin/sh");

Thus, patch the occurrences of "/bin/sh" in the source files of Emacs to refer
to their correct store location.

* gnu/packages/emacs.scm (emacs)[phases]{patch-program-file-names}: Add phase.

gnu/packages/emacs.scm

@@ -131,6 +131,16 @@ (define-public emacs
                                "--disable-build-details")
        #:phases
        (modify-phases %standard-phases
+         (add-after 'unpack 'patch-program-file-names
+           (lambda _
+             (substitute* '("src/callproc.c"
+                            "lisp/term.el"
+                            "lisp/htmlfontify.el"
+                            "lisp/textmodes/artist.el"
+                            "lisp/progmodes/sh-script.el")
+               (("\"/bin/sh\"")
+                (format "~s" (which "sh"))))
+             #t))
          (add-before 'configure 'fix-/bin/pwd
            (lambda _
              ;; Use `pwd', not `/bin/pwd'.