nixos/atop: Fix regression in enabling atop units

[tollb]

Fix regression where the systemd units for atop are no longer automatically started at boot when programs.atop.enable = true.

Regression was introduced in commit: 09350ff
nixos/atop: Convert log format to fix service start

This commit restructures the atop systemd service config so that the code to convert the log format gets configured as a preStart script along with the addition of the wantedBy rule.

Additional notes on the regression:
The atop unit files get created correctly, but are not linked from
/etc/systemd/system/multi-user.target.wants.

With the module's default settings, the two units that are not started
are: atop.service and atopacct.service. As a result, atop logs do not
get created under /var/log/atop, unless the units are started manually.

The associated PR: #175558

The problem with the units not starting is noted in a comment, along
with a workaround, on PR: #175558 (comment)

Additional notes on testing:
To test, I ran the atop.defaults test (which had been failing on master):

$ nix-build '<nixpkgs>' -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/c7e4b0832c92b56db5916811083d2c6963e0bb2d.tar.gz -A nixosTests.atop.defaults

20230708 -- updated test URL for latest commit:

$ nix-build '<nixpkgs>' -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/8f4f1ce005ceba182805d5a8866e39269b9a23a5.tar.gz -A nixosTests.atop.defaults

I also successfully built nixosTests.atop.netatop, which had also been failing.

I also built nixosTests.atop.justThePackage and nixosTests.atop.minimal, which had not been failing.

I skipped testing of nixosTests.atop.atopgpu and nixosTests.atop.everything.

NOTE: I believe the update can be backported to NixOS 23.05.

@pschyska @7c6f434c @klemensn

Description of changes

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.11 Release Notes (or backporting 23.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/prs-ready-for-review/3032/2402

[pschyska]

Thanks for this @tollb .
I finally got around testing it by removing my local fixes, and it seems to work fine!