Allow substituting paths when building remotely using ssh-ng://

[Ma27]

Until now, it was not possible to substitute missing paths from e.g.
https://cache.nixos.org on a remote server when building on it using
the new ssh-ng protocol.

This is because every store implementation except legacy ssh://
ignores the substitution flag passed to Store::queryValidPaths while
the legacy-ssh-store substitutes the remote store using
cmdQueryValidPaths when the remote store is opened with nix-store --serve.

This patch slightly modifies the daemon protocol to allow passing an
integer value suggesting whether to substitute missing paths during
wopQueryValidPaths. To implement this on the daemon-side, the
substitution logic from nix-store --serve has been moved into a
protected method named Store::substitutePaths which gets currently
called from LocalStore::queryValidPaths and Store::queryValidPaths
if maybeSubstitute is true.

Fixes #2770

cc @elaforge @edolstra @Ericson2314 @lheckemann

[Ericson2314]

Excellent change!

[Ma27]

@edolstra does this look OK to you as well? :)

[Ma27]

ping @edolstra anything missing to get this merged? %)

[edolstra]

maybeSubstitute is a pretty ugly hack and it would probably be better to get rid of it somehow rather than add it to other stores. For instance, nix copy could call destStore->substitutePaths() (or destStore->buildPaths()) directly.

[Ma27]

@edolstra so you mean that the daemon (and store) workers should invoke destStore->substitutePaths() directly if needed?

[Ma27]

@edolstra the Store::substitutePaths() method is now called from nix-daemon directly. Anything else to fix to get this merged? :)

[Ma27]

ping @edolstra anything else tbd here to get this merged? :)

[Ma27]

@edolstra any updates here? :)

[edolstra]

Thanks!