.github: clarify things done re: sandboxing
.github: clarify things done re: sandboxing

Per https://github.com/NixOS/nixpkgs/pull/58147#issuecomment-478234495
sandboxing is not fully functional on macOS.

.github: specify where sandboxing can be run

Co-Authored-By: Jon <jonringer@users.noreply.github.com>

.github: remove macOS note

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