[Chore] Run CI checks on 'pull_request'

Problem: We want to be able to run CI checks on PRs from external forks.
However, this is only possible with 'on: pull_request', while currently
CI is triggered 'on: push'

Solution: Change CI triggering condition to 'on: pull_request'.

Replace jsonschema-cli with check-jsonschema

jsonschema-cli is deprecated and will be removed in the future.
The recommended replacement is check-jsonschema.

[#201] Deduce profile directory during activation

Problem: Since https://github.com/NixOS/nix/pull/5226 nix profiles for
users are stored in 'XDG_STATE_HOME' or 'HOME' directory. However,
'deploy-rs' still expects profiles to be present in
'/nix/var/nix/profiles/per-user'. As a result, an attempt to deploy a
profile with newer nix may fail with an error about non-existing files.

Solution: Instead of deducing the profile path prior to ssh'ing and
actual activation, deduce the path to the profile during as a part of
'activate-rs' invocation.

Now if the profile path is not specified explicitly as an attribute in
profile within the deploy flake, the path to the profile is determined
based on the user to which the profile belongs and on the values of
'XDG_STATE_HOME' and 'HOME' variables.
Additionally, if the old profile directory (in
'/nix/var/nix/profiles/per-user') for a given user already exists, it is
used instead for the sake of backward compatibility.

Replace "eachother" with "each other".

actually merge confirm_timeout into merged_settings

[#210] Add activation script for darwin system and provide a usage example