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.

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'.

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

Problem: README misses documentation for 'confirmTimeout' and
'activationTimeout'.

Solution: Mention them in README.

Problem: When profile activation confirmation fails due to
confirmation timeout and performs a rollback, zero exit code is
returned. Such a behavior is confusing since rollback usually means
something went wrong during deployment and it shouldn't return
successful exit code.

Solution: Explicitly return confirmation waiting error instead of
printing it and silently signalizing success.

Problem: Currently profile activation waiting timeout is hardcoded to
240 seconds, see https://github.com/serokell/deploy-rs/pull/48.
In some cases, this timeout can be exceeded (e.g.
activation performs a heavy DB migration and waits for it to finish
before considering the profile activation succesful).

Solution: Make this timeout configurable via 'activationTimeout' deploy
attribute or corresponding '--activation-timeout' CLI option. For the
sake of backward compatibility, the new 'wait' subcommand
'--activation-timeout' option is made optional and defaults to 240
seconds if it wasn't provided.

closes: #241