Nixpkgs: Update pull request template to include nix show-config for sandbox check in 18.09

[Mic92]

Issue description

it should contain something like:

$ nix show-config | grep 'sandbox ='
sandbox = true

to figure out if sandbox builds are enabled.

[vcunat]

I don't think we can assume the nix command yet. (for a few months)

[dtzWill]

Would this be better than using nix-info? Or perhaps this is something nix-info can leverage?

[Mic92]

nix-info could be also mentioned instead of showing configuration values.

[teto]

Also if you update the template, please put the advice as a markdown comment so that we don't have to remove it when filing the issue (the comment will remain in the issue but won't be displayed by github).

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