oss-fuzz: build is broken #33232

Now that #31802 is merged, oss-fuzz needs fixing, to either opt-out of multiprocess, or configure depends to use the pre-built Clang for native package compilation (as Ubuntu 20.04 ships with GCC 9):

/src/bitcoin-core/depends/work/download/native_capnp-1.2.0/capnproto-cxx-1.2.0.tar.gz.temp: OK
Extracting native_capnp...
/src/bitcoin-core/depends/sources/capnproto-cxx-1.2.0.tar.gz: OK
Preprocessing native_capnp...
Configuring native_capnp...
-- The CXX compiler identification is GNU 9.4.0
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - failed
-- Check for working CXX compiler: /usr/bin/g++
-- Check for working CXX compiler: /usr/bin/g++ - broken
CMake Error at /usr/local/share/cmake-3.29/Modules/CMakeTestCXXCompiler.cmake:60 (message):
  The C++ compiler

    "/usr/bin/g++"

  is not able to compile a simple test program.

  It fails with the following output:

    Change Dir: '/src/bitcoin-core/depends/work/build/x86_64-pc-linux-gnu/native_capnp/1.2.0-b8ef312d3bf/CMakeFiles/CMakeScratch/TryCompile-8xnY4X'
    
    Run Build Command(s): /usr/local/bin/cmake -E env VERBOSE=1 /usr/bin/make -f Makefile cmTC_a7435/fast
    make[1]: Entering directory '/src/bitcoin-core/depends/work/build/x86_64-pc-linux-gnu/native_capnp/1.2.0-b8ef312d3bf/CMakeFiles/CMakeScratch/TryCompile-8xnY4X'
    /usr/bin/make  -f CMakeFiles/cmTC_a7435.dir/build.make CMakeFiles/cmTC_a7435.dir/build
    make[2]: Entering directory '/src/bitcoin-core/depends/work/build/x86_64-pc-linux-gnu/native_capnp/1.2.0-b8ef312d3bf/CMakeFiles/CMakeScratch/TryCompile-8xnY4X'
    Building CXX object CMakeFiles/cmTC_a7435.dir/testCXXCompiler.cxx.o
    /usr/bin/g++   -I/src/bitcoin-core/depends/x86_64-pc-linux-gnu/native/include       -pipe -std=c++20  -fPIE -o CMakeFiles/cmTC_a7435.dir/testCXXCompiler.cxx.o -c /src/bitcoin-core/depends/work/build/x86_64-pc-linux-gnu/native_capnp/1.2.0-b8ef312d3bf/CMakeFiles/CMakeScratch/TryCompile-8xnY4X/testCXXCompiler.cxx
    g++: error: unrecognized command line option '-std=c++20'; did you mean '-std=c++2a'?
    make[2]: *** [CMakeFiles/cmTC_a7435.dir/build.make:78: CMakeFiles/cmTC_a7435.dir/testCXXCompiler.cxx.o] Error 1

See also #31802 (comment).

Since I'm not at all familiar with oss-fuzz, I've gone for the simplest solution for now: https://github.com/google/oss-fuzz/pull/13854

Once that's merged, let's rename this issue to "oss-fuzz: IPC support disabled", because I do think we should add coverage.

should be fixed?