ci: Fix shell.nix compatibility with CMake 4.0

CMake 4.0 dropped support for older policies, so versions of Cap'n Proto <=
v1.1.0 (before capnproto/capnproto#2272) require an
additional option to build with CMake 4.0. Without this change, there are
errors like:

> CMake Error at CMakeLists.txt:1 (cmake_minimum_required):
>   Compatibility with CMake < 3.5 has been removed from CMake.
>
>   Update the VERSION argument <min> value.  Or, use the <min>...<max> syntax
>   to tell CMake that the project requires at least <min> but has been updated
>   to work with policies introduced by <max> or earlier.
>
>   Or, add -DCMAKE_POLICY_VERSION_MINIMUM=3.5 to try configuring anyway.

This change fixes the problem by specifying -DCMAKE_POLICY_VERSION_MINIMUM=3.5
when needed.

Author: ryanofsky

shell.nix

@@ -39,6 +39,7 @@ let
       hash  = lib.attrByPath [capnprotoVersion] "" capnprotoHashes;
     };
     patches = lib.optionals (lib.versionAtLeast capnprotoVersion "0.9.0" && lib.versionOlder capnprotoVersion "0.10.4") [ ./ci/patches/spaceship.patch ];
+    cmakeFlags = (old.cmakeFlags or []) ++ (lib.optionals (lib.versionAtLeast "1.1.0" capnprotoVersion) ["-DCMAKE_POLICY_VERSION_MINIMUM=3.5"]);
   } // (lib.optionalAttrs (lib.versionOlder capnprotoVersion "0.10") {
     env = { }; # Drop -std=c++20 flag forced by nixpkgs
   }));