build-system/meson: Really skip the 'fix-runpath' phase on armhf.

This follows up commit d5b5a15a40 which doesn't
work because %current-system etc expands before the actual build.

Fixes <https://bugs.gnu.org/31719>.

* guix/build-system/meson.scm (meson-build)[builder]: Compare against the
already existing "system" variable rather than (%current-system).

guix/build-system/meson.scm

@@ -148,8 +148,7 @@ has a 'meson.build' file."
                     #:search-paths ',(map search-path-specification->sexp
                                           search-paths)
                     #:phases
-                    (if (string-prefix? "arm" ,(or (%current-target-system)
-                                                   (%current-system)))
+                    (if (string-prefix? "arm" ,system)
                         (modify-phases build-phases (delete 'fix-runpath))
                         build-phases)
                     #:configure-flags ,configure-flags