nixos/phpfpm: allow configuring php.ini files per-pool

[aanderse]

Motivation for this change

A follow up to #41440 which then allows you to specify different php.ini files per pool. Also fixes a bug where the php.ini which is supplied to a pool isn't necessarily a php.ini from the same version of php.
The nextcloud module sets some php.ini options which are applied to all pools. If accepted this PR would allow the nextcloud modules to set options in php.ini which don't impact other pools on the system, truly sandboxing it and allowing for much more reproducible systems.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

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[aanderse]

@GrahamcOfBorg test nextcloud

[aanderse]

@wmertens as this is a direct continuation of your PR any thoughts?

[jtojnar]

Thanks.