Pull request template: comment out instructions

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.github
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.github/PULL_REQUEST_TEMPLATE.md
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###### Things done
-
Please check what applies. Note that these are not hard requirements but merely serve as information for reviewers.
- [ ] Tested using sandboxing
([nix.useSandbox](http://nixos.org/nixos/manual/options.html#opt-nix.useSandbox) on NixOS,
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###### Things done
+
<!-- Please check what applies. Note that these are not hard requirements but merely serve as information for reviewers. -->
- [ ] Tested using sandboxing
([nix.useSandbox](http://nixos.org/nixos/manual/options.html#opt-nix.useSandbox) on NixOS,