nix: make dist tarball in nix

This is more sound than having to rely on host nix shell.

Signed-off-by: Ophestra <cat@gensokyo.uk>

flake.nix

@@ -67,17 +67,25 @@
       packages = forAllSystems (
         system:
         let
-          inherit (self.packages.${system}) hakurei-static caddy-hakurei-static;
+          inherit (self.packages.${system}) static caddy;
           pkgs = nixpkgsFor.${system};
         in
         {
-          default = caddy-hakurei-static;
-          hakurei-static = pkgs.callPackage ./package.nix { };
-          caddy-hakurei-static = pkgs.writeShellScriptBin "caddy-hakurei-static" ''
+          default = caddy;
+          static = pkgs.callPackage ./package.nix { };
+          dist = pkgs.runCommand "hakurei-static-${static.version}.tar.zst" { } ''
+            PATH="${pkgs.zstd}/bin:$PATH" \
+              ${pkgs.gnutar}/bin/tar \
+              -C '${static}' \
+              --zstd \
+              -cf \
+              "$out" .
+          '';
+          caddy = pkgs.writeShellScriptBin "caddy-hakurei-static" ''
             exec ${pkgs.caddy}/bin/caddy \
               file-server \
               -a -l ":49151" \
-              -r ${hakurei-static}
+              -r ${static}
           '';
         }
       );