Merge pull request #253973 from trofi/bcache-make-optional

- Certificate generation via the `security.acme` now limits the concurrent number of running certificate renewals and generation jobs, to avoid spiking resource usage when processing many certificates at once. The limit defaults to *5* and can be adjusted via `maxConcurrentRenewals`. Setting it to *0* disables the limits altogether.

- New `boot.bcache.enable` (default enabled) allows completely removing `bcache` mount support.

## Nixpkgs internals {#sec-release-23.11-nixpkgs-internals}

- The use of `sourceRoot = "source";`, `sourceRoot = "source/subdir";`, and similar lines in package derivations using the default `unpackPhase` is deprecated as it requires `unpackPhase` to always produce a directory named "source". Use `sourceRoot = src.name`, `sourceRoot = "${src.name}/subdir";`, or `setSourceRoot = "sourceRoot=$(echo */subdir)";` or similar instead.

{ config, lib, pkgs, ... }:

{

{ config, lib, pkgs, ... }: let

  cfg = config.boot.bcache;

in {

  options.boot.bcache.enable = lib.mkEnableOption (lib.mdDoc "bcache mount support") // {

    default = true;

    example = false;

  };

  options.boot.initrd.services.bcache.enable = lib.mkEnableOption (lib.mdDoc "bcache support in the initrd") // {

    description = lib.mdDoc ''

      *This will only be used when systemd is used in stage 1.*

    '';

  };

  config = {

  config = lib.mkIf cfg.enable {

    environment.systemPackages = [ pkgs.bcache-tools ];