tests.cross.sanity: enable mbuffer test

This test passes now.  Also fixes a minor oversight in the bug --
the test case needs to `touch $out` on success.

pkgs/test/cross/default.nix

@@ -115,6 +115,7 @@ let
   in
     pkgs.runCommand "test-mbuffer" {} ''
       echo hello | ${emulator} ${mbuffer}/bin/mbuffer
+      touch $out
     '';
 
   # This is meant to be a carefully curated list of builds/packages
@@ -127,7 +128,7 @@ let
   # of things that often break.  So, no buckshot `mapTestOnCross`
   # calls here.
   sanity = [
-    #pkgs.mbuffer              # https://github.com/NixOS/nixpkgs/issues/213453
+    mbuffer
     #pkgs.pkgsCross.gnu64.bash # https://github.com/NixOS/nixpkgs/issues/243164
     pkgs.gcc_multi.cc
     pkgs.pkgsMusl.stdenv