disable BUILD_TESTING for capnproto

[n-west]

Running in a colab notebook & docker based on nvidia/cuda:12.2.0-devel-ubuntu22.04 I hit the following error building:

Executing subproject capnproto method cmake 

| Configuring the build directory with CMake version 3.22.1
| Running CMake with: -G Ninja -DCMAKE_INSTALL_PREFIX=/usr/local -DCMAKE_POSITION_INDEPENDENT_CODE=True -D-BUILD_TESTING=False
|   - build directory:          /seticore/build/subprojects/capnproto/__CMake_build
|   - source directory:         /seticore/subprojects/capnproto
|   - toolchain file:           /seticore/build/subprojects/capnproto/__CMake_build/CMakeMesonToolchainFile.cmake
|   - preload file:             /usr/local/lib/python3.10/dist-packages/mesonbuild/cmake/data/preload.cmake
|   - trace args:               --trace-expand --trace-format=json-v1 --no-warn-unused-cli --trace-redirect=cmake_trace.txt
|   - disabled policy warnings: [CMP0025, CMP0047, CMP0056, CMP0060, CMP0065, CMP0066, CMP0067, CMP0082, CMP0089, CMP0102]

| Running with expanded trace output on.
| Not searching for unused variables given on the command line.
| Trace will be written to cmake_trace.txt
| -- The CXX compiler identification is GNU 11.4.0
| -- Detecting CXX compiler ABI info
| -- Detecting CXX compiler ABI info - done
| -- Check for working CXX compiler: /usr/bin/c++ - skipped
| -- Detecting CXX compile features
| -- Detecting CXX compile features - done
| -- Looking for C++ include initializer_list
| -- Looking for C++ include initializer_list - found
| -- Found OpenSSL: /usr/lib/x86_64-linux-gnu/libcrypto.so (found version "3.0.2") found components: Crypto SSL
| -- Looking for makecontext in c
| -- Looking for makecontext in c - found
| -- Found ZLIB: /usr/lib/x86_64-linux-gnu/libz.so (found version "1.2.11")
| -- Configuring done
| -- Generating done
| -- Build files have been written to: /seticore/build/subprojects/capnproto/__CMake_build

capnproto| CMake configuration: SUCCEEDED
capnproto| WARNING: Unable to evaluate the cmake variable '$<TARGET_FILE:capnp_tool>'.
capnproto| WARNING: Unable to evaluate the cmake variable '$<TARGET_FILE:capnp_tool>'.
capnproto| WARNING: Unable to evaluate the cmake variable '$<TARGET_FILE:capnp_tool>'.
capnproto| WARNING: Unable to evaluate the cmake variable '$<TARGET_FILE:capnpc_cpp>'.

meson.build:37:22: ERROR: We evaluated the cmake variable '$<TARGET_FILE:capnp_tool>' to an empty string, which is not a valid path to an executable.

Disabling the testing build of capnproto appears to resolve the issue

[lacker]

LGTM