gnu: gnulib: Refer to "gcc" as provided via implicit package inputs.

Fixes <https://issues.guix.gnu.org/60947>.

Previously, with the #+(file-append gcc "/bin/gcc") reference, the
resulting derivation would end up referring to the grafted GCC when
grafts are enabled, and to the ungrafted one otherwise.  As a result, a
different derivation would be produced depending on whether grafts are
enabled.

* gnu/packages/build-tools.scm (gnulib-checkout)[arguments]: Refer to
"gcc", not #+(file-append gcc "/bin/gcc").

gnu/packages/build-tools.scm

@@ -929,7 +929,7 @@ Makefiles, JSON Compilation Database, and experimentally Ninja.")
               (with-directory-excursion "lib"
                 ;; See the compile-command buffer-local variable in
                 ;; lib/gen-uni-tables.c
-                (invoke #+(file-append gcc "/bin/gcc") "-O" "-Wall" "gen-uni-tables.c"
+                (invoke "gcc" "-O" "-Wall" "gen-uni-tables.c"
                         "-Iunictype" "-o" "gen-uni-tables")
                 (apply invoke
                        "./gen-uni-tables"