build: check-final-inputs-self-contained uses %HYDRA-SUPPORTED-SYSTEMS.

* build-aux/check-final-inputs-self-contained.scm: Change
  '%supported-systems' to '%hydra-supported-systems'.

build-aux/check-final-inputs-self-contained.scm

@@ -77,5 +77,5 @@ (define (test-final-inputs store system)
     (set-build-options store #:use-substitutes? #t)
 
     (for-each (cut test-final-inputs store <>)
-              %supported-systems)))
+              %hydra-supported-systems)))