This removes the dependency on the `nixpkgs` channel within the
modules for state version ≥ 20.09. The default Nixpkgs source starting
from this state version is the path of the `pkgs` argument used to
bootstrap the Home Manager modeuls.
This is a prerequisite for using Home Manager withing Nix flakes.
PR #1420
The git-send-email [0] script uses StartTLS if `smtpEncryption` is set
to `tls`, which can break services that don't support StartTLS.
[0]: bd42bbe1a4/git-send-email.perl (L1533)
PR #1395
This makes the systemd module use the sd-switch application to perform
the unit switch during a generation activation.
Since the closure of sd-switch is relatively lightweight we
unconditionally pull it in as a dependency. We simultaneously remove
the `systemd.user.startServices` option and perform the switch action
automatically.
PR #1388
