PULL_REQUEST_TEMPLATE.md: Improve platform checkboxes

1) Building with the sandbox enabled is the standard on Linux,
   so it is of little relevance with distribution they are using.
   Because of this, we now specify "Linux" instead of "NixOS",
   and the sandbox checkbox specifies "non-Linux".
2) aarch64 is a much more common platform, so we add two separate
   checkboxes for both aarch64-linux and aarch64-darwin.
3) The platform names now match what is actually used by Nix.

.github/PULL_REQUEST_TEMPLATE.md

@@ -15,11 +15,12 @@ Reviewing guidelines: https://nixos.org/manual/nixpkgs/unstable/#chap-reviewing-
 
 <!-- Please check what applies. Note that these are not hard requirements but merely serve as information for reviewers. -->
 
-- [ ] Tested using sandboxing ([nix.useSandbox](https://nixos.org/nixos/manual/options.html#opt-nix.useSandbox) on NixOS, or option `sandbox` in [`nix.conf`](https://nixos.org/nix/manual/#sec-conf-file) on non-NixOS linux)
 - Built on platform(s)
-   - [ ] NixOS
-   - [ ] macOS
-   - [ ] other Linux distributions
+  - [ ] x86_64-linux
+  - [ ] aarch64-linux
+  - [ ] x86_64-darwin
+  - [ ] aarch64-darwin
+- [ ] For non-Linux: Is `sandbox = true` set in `nix.conf`? (See [Nix manual](https://nixos.org/manual/nix/stable/#sec-conf-file))
 - [ ] Tested via one or more NixOS test(s) if existing and applicable for the change (look inside [nixos/tests](https://github.com/NixOS/nixpkgs/blob/master/nixos/tests))
 - [ ] Tested compilation of all packages 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/`)