github issue template: add sandboxing info

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.github
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.github/ISSUE_TEMPLATE.md
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* System: (NixOS: `nixos-version`, Ubuntu/Fedora: `lsb_release -a`, ...)
* Nix version: (run `nix-env --version`)
* Nixpkgs version: (run `nix-instantiate --eval '<nixpkgs>' -A lib.nixpkgsVersion`)
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* Sandboxing enabled: (run `grep build-use-sandbox /etc/nix/nix.conf`)