freshBootstrapTools: enable musl on RISC-V

musl now supports RISC-V.  Let's centralise musl availability checks
in musl.meta.platforms, so we don't have to keep cleaning up ad-hoc
checks like this all over the tree.

pkgs/stdenv/linux/make-bootstrap-tools.nix

@@ -20,7 +20,7 @@ in with pkgs; rec {
   tarMinimal = gnutar.override { acl = null; };
 
   busyboxMinimal = busybox.override {
-    useMusl = !stdenv.targetPlatform.isRiscV;
+    useMusl = lib.meta.availableOn stdenv.hostPlatform musl;
     enableStatic = true;
     enableMinimal = true;
     extraConfig = ''