This adds a --file option, which causes deploy-rs to always use its
non-flake logic. Before this change, we would ask nix if it understood
flakes, and only if not fall back to the non-flake logic. This has
utility assuming one has a nix flake repository with flake-compat set
up, but prevents use with alternative source-managing tools such as niv
or npins if nix has the "flakes" experimental feature enabled.

doc: add option doc for interactiveSudo

fix nix-build invocation in non-flake builds

this adds a nixos vm test doing a deploy on a nix which does not have
flakes enabled, to guard against this breaking as it has done before [1].

The existing test infrastructure is changed slightly to make enabling
flakes configurable inside the vm's config.

[1] https://github.com/serokell/deploy-rs/pull/272

add meta.mainProgram to rust drv

The drvpath^out syntax is only part of `nix build', not `nix-build',
which still produces the outPath as it did before, but errors out if
attempted to be called in the same way as `nix build'.

better error messages: provide node names

printing what node caused an error is very useful for debugging
build or deployment failures, especially when deploying to several
nodes at the same time