php80Extensions.apcu_bc: mark as broken

[Ma27]

Motivation for this change

ZHF #122042
Failing Hydra build: https://hydra.nixos.org/build/142591498

apcu_bc[1] is a PHP extension which provides support for the older APC
caching-interface (i.e. apc_fetch, apc_store etc.) on newer PHP
versions. As the codebase wasn't touched since 2019 it's not really
surprising that it doesn't compile for PHP 8. Also, if people have
already managed to get their codebases running on PHP8, they're most
likely also using the newer APCu[2] interface.

I think it's okay to keep this as long as we support PHP 7.4 and drop
the package after that.

[1] https://pecl.php.net/package/apcu_bc
[2] https://www.php.net/manual/en/book.apcu.php

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• 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 nixpkgs-review --run "nixpkgs-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)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.