ci: disable -Werror=maybe-uninitialized for Windows builds

This produces false positives, such as:
```bash
torcontrol.cpp: In static member function ‘static void TorControlConnection::readcb(bufferevent*, void*)’:
torcontrol.cpp:94:28: error: ‘result’ may be used uninitialized [-Werror=maybe-uninitialized]
   94 |         self->message.code = ToIntegral<int>(s.substr(0, 3)).value_or(0);
      |         ~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from ./netaddress.h:18,
                 from ./torcontrol.h:11,
                 from torcontrol.cpp:6:
./util/strencodings.h:184:7: note: ‘result’ was declared here
  184 |     T result;
      |       ^~~~~~
cc1plus: all warnings being treated as errors
make[2]: *** [Makefile:11088: libbitcoin_node_a-torcontrol.o] Error 1
```

ci/test/00_setup_env_win64.sh

@@ -13,4 +13,4 @@ export DPKG_ADD_ARCH="i386"
 export PACKAGES="nsis g++-mingw-w64-x86-64-posix wine-binfmt wine64 wine32 file"
 export RUN_FUNCTIONAL_TESTS=false
 export GOAL="deploy"
-export BITCOIN_CONFIG="--enable-reduce-exports --enable-external-signer --disable-gui-tests"
+export BITCOIN_CONFIG="--enable-reduce-exports --enable-external-signer --disable-gui-tests CXXFLAGS='-Wno-error=maybe-uninitialized'"