matomo: ignore prereleases in updateScript and make src overridable

[niklaskorz]

The update script now does its job as seen in #378885, but currently also updates to pre-releases. This is fixed by adding the --version-regex parameter to the update script.

I also noticed today that fetchurl is not overrideable by default (see #158018), which makes the matomo-beta example in #374022 not work.
This is fixed by applying lib.makeOverridable to fetchurl.

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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[Ma27]

Any reason fetchurl isn't overridable in the first place?

[niklaskorz]

There is a pending PR in #158018

[niklaskorz]

To be clear, I don't exactly want fetchurl' = lib.makeOverridable fetchurl to become a widespread pattern, but seeing we removed matomo-beta in unstable, I think in this particular case we still want to make it easy to override the matomo src before that PR lands.