Use system zstd instead of the bundled one

The bundled zstd is currently broken, use the system one for now.

pkgs/nix-portable/default.nix

@@ -236,7 +236,7 @@ let
       ${installBin nixStatic "nix"}
 
       # install ssl cert bundle
-      unzip -poj "\$self" ${ lib.removePrefix "/" "${caBundleZstd}"} | \$dir/bin/zstd -d > \$dir/ca-bundle.crt
+      unzip -poj "\$self" ${ lib.removePrefix "/" "${caBundleZstd}"} | /usr/bin/zstd -d > \$dir/ca-bundle.crt
 
       recreate_nix_conf
     fi
@@ -472,7 +472,7 @@ let
         rm -rf \$dir/tmp/*
         cd \$dir/tmp
         unzip -qqp "\$self" ${ lib.removePrefix "/" "${storeTar}/tar"} \
-          | \$dir/bin/zstd -d \
+          | /usr/bin/zstd -d \
           | tar -x \$missing --strip-components 2
         mv \$dir/tmp/* \$store/
       )