populate flake: add fetchOnTarget

With fetchOnTarget, the target fetches the input and its own inputs itself,
using fetchTree with the arguments recorded in the flake's lock file.  Only
nix flake metadata runs on the deploying machine, so the inputs get
downloaded once, by the target.  This needs Nix with flake support on the
target, which populate checks before fetching.

Because nothing gets copied, this also works for targets using sudo.

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