Signed-off-by: Henri Rosten <henri.rosten@unikie.com>

Runs the build with cachix watch-exec to start pushing nix store
paths to the cache in the background already during the build, so
other build jobs could re-use them while still building.

Signed-off-by: Henri Rosten <henri.rosten@unikie.com>

- Start-off with just two build targets.
- Rename the cachix authentication token.
- Replace cachix cache substituter with the 'official' non-trial
  version.
- Remove debug steps: printing nix.conf and timing the build
  commands.
- Remove 'workflow_dispatch' event from the workflow triggers, to
  *not* allow manually re-triggering the action.

Signed-off-by: Henri Rosten <henri.rosten@unikie.com>