gnu: commencement: Make ‘glibc-final’ immune to ‘%current-target-system’.

Previously, the value memoized for ‘glibc-final’ was influenced by that
of ‘%current-target-system’ since it is used as a default argument to
‘libc-for-target’.  Thus, due to the order in which modules were loaded
versus the time at which ‘%current-target-system’ was set, it was
possible to end up with ‘glibc-final’ set to the “wrong” libc, for
instance when doing:

  guix build -f guix.scm --target=i586-pc-gnu

from another project such as the Shepherd.

This is similar to the problem fixed in
625a847d2a.

* gnu/packages/commencement.scm (glibc-final-with-bootstrap-bash)
(glibc-final): Explicitly pass (%current-system) as the first argument
to ‘libc-for-target’ so that the value of ‘%current-target-system’ at
that time has no influence.

Change-Id: I2f05898b8f9d1f99d4c75108441c7ad38728225f

gnu/packages/commencement.scm

@@ -2922,7 +2922,7 @@ memoized as a function of '%current-system'."
   ;; store path has no dependencies.  Actually, the really-final libc is
   ;; built just below; the only difference is that this one uses the
   ;; bootstrap Bash.
-  (let ((libc (libc-for-target)))
+  (let ((libc (libc-for-target (%current-system))))
     (package
       (inherit libc)
       (name "glibc-intermediate")
@@ -3096,7 +3096,7 @@ exec ~a/bin/~a-~a -B~a/lib -Wl,-dynamic-linker -Wl,~a/~a \"$@\"~%"
 (define/system-dependent glibc-final
   ;; The final glibc, which embeds the statically-linked Bash built above.
   ;; Use 'package/inherit' so we get the 'replacement' of 'glibc', if any.
-  (let ((libc (libc-for-target)))
+  (let ((libc (libc-for-target (%current-system))))
     (package/inherit libc
       (name "glibc")
       (source (bootstrap-origin (package-source libc)))