buildah-unwrapped: fix cross compilation

pkgs/development/tools/buildah/default.nix

···
     runHook preBuild
     patchShebangs .
     make bin/buildah
-    make -C docs GOMD2MAN="${go-md2man}/bin/go-md2man"
+    make -C docs GOMD2MAN="go-md2man"
     runHook postBuild
   '';