nixos/matomo: fix service failure when not fully set up

[osnyx]

Since matomo-5.2.0, the config.php.ini is already created when first
accessing the installer page without completing it. This breaks our
discovery of whether to run database migrations.

Attempting to run DB migrations without provided database credentials
causes a crash -> causing matomo-setup-update.service to fail -> causing
phpfpm-matomo.service to fail.
This is a problem when the service (or the whole system) is restarted during the start, but before the end of the setup process of the web installer. With the phpfpm-matomo.service failing to start due to a failing dependency, finishing up the setup process won't be possible anymore.

I have also extended the corresponding NixOS tests to cover aspects we have already been covering in a more detailed downstream test for our own matomo module.
The last test with the systemctl kill was actually the one revealing the issue fixed here.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• 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/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 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.

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