nix copy: Add --profile and --out-link flags

[edolstra]

Motivation

This allows nix copy to atomically copy a store path and point a profile to it, without the risk that the store path might be GC'ed in between. This is useful for instance when deploying a new NixOS system profile from a remote store.

Context

Priorities and Process

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[fricklerhandwerk]

Needs release notes and some way to document in the command reference that this will exhibit the behavior outlined in the PR description. Right now --profile just says "The profile to operate on", and that will show on every command that uses the mixin - not very informative.

The cheap way to add the relevant docs for this command is adding an appropriate blurb to the new example. Ideally though we'd have a way to annotate what a flag, which enabled by a mixin, means for each command that uses it. Probably overkill, but mixins seem to add a set of possible behaviors, so we'd need to describe which behaviors are used how.

[edolstra]

@fricklerhandwerk I agree, but that's a bit of a general problem beyond the scope of this PR, since currently every MixProfile command has this issue.

What do you mean with "appropriate blurb"?

[edolstra]

#11659 for the --profile documentation issue.