gnu: docker: Fix build on btrfs.

* gnu/packages/docker.scm (docker)[arguments]: Skip another test that
requires root privileges.

gnu/packages/docker.scm

@@ -397,6 +397,7 @@ (define-public docker
              ;; Timeouts after 5 min.
              (delete-file "plugin/manager_linux_test.go")
              ;; Operation not permitted.
+             (delete-file "daemon/graphdriver/btrfs/btrfs_test.go")
              (delete-file "daemon/graphdriver/overlay/overlay_test.go")
              (delete-file "daemon/graphdriver/overlay2/overlay_test.go")
              #t))