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.

This problem does not affect the olddeps job because olddeps uses an old
version of Cap'n Proto together with an old version of cmake. But in a custom
config, specifying an older capnprotoVersion argument to shell.nix would cause
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
   }));