spago: fix Darwin build

[simonchatts]

Motivation for this change

As noted earlier, Spago is currently broken for Darwin. This fixes that.

It's a weird issue. The problem is that when doing a fileEmbed of the content downloaded at build time, read permissions are insufficient. dtruss confirms that ghc requests write permissions when calling open.

I don't understand why this should happen, but all the static files in the same templates directory have permissions 0644, and making it the same for the two separately downloaded files seems benign enough as a workaround. Once the build is complete, it all gets deleted anyway.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[cdepillabout]

Seems like a reasonable workaround. Thanks for taking a look at this.