nix run: Add some flags for clearing/keeping the environment
nix run: Add some flags for clearing/keeping the environment

This is useful for testing commands in isolation.

For example,

  $ nix run nixpkgs.geeqie -i -k DISPLAY -k XAUTHORITY -c geeqie

runs geeqie in an empty environment, except for $DISPLAY and
$XAUTHORITY.

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