ci: Disable external signer in native macOS CI job

[hebasto]

Boost.Process 1.88 currently fails the configure test:

2025-05-06T15:12:35.6692210Z configure:37666: checking whether Boost.Process can be used
2025-05-06T15:12:35.6693270Z configure:37697: g++ -std=gnu++11 -std=c++17 -o conftest -g -O2 -Wno-error=narrowing  -isystem /usr/local/include -DBOOST_MULTI_INDEX_DISABLE_SERIALIZATION  -pthread conftest.cpp  >&5
2025-05-06T15:12:35.6694210Z conftest.cpp:82:9: error: no type named 'opstream' in namespace 'boost::process'
2025-05-06T15:12:35.6694340Z     bp::opstream stdin_stream;
2025-05-06T15:12:35.6694430Z     ~~~~^
2025-05-06T15:12:35.6694890Z conftest.cpp:83:18: error: use of undeclared identifier 'stdout_stream'; did you mean 'stdin_stream'?
2025-05-06T15:12:35.6695030Z     bp::ipstream stdout_stream;
2025-05-06T15:12:35.6695120Z                  ^~~~~~~~~~~~~
2025-05-06T15:12:35.6695220Z                  stdin_stream
2025-05-06T15:12:35.6695440Z conftest.cpp:82:18: note: 'stdin_stream' declared here
2025-05-06T15:12:35.6695560Z     bp::opstream stdin_stream;
2025-05-06T15:12:35.6695650Z                  ^
2025-05-06T15:12:35.6695870Z conftest.cpp:83:17: error: expected ';' after expression
2025-05-06T15:12:35.6696000Z     bp::ipstream stdout_stream;
2025-05-06T15:12:35.6696090Z                 ^
2025-05-06T15:12:35.6696180Z                 ;
2025-05-06T15:12:35.6696530Z conftest.cpp:83:9: error: no member named 'ipstream' in namespace 'boost::process'
2025-05-06T15:12:35.6696650Z     bp::ipstream stdout_stream;
2025-05-06T15:12:35.6696740Z     ~~~~^
2025-05-06T15:12:35.6696960Z conftest.cpp:84:14: error: expected ';' after expression
2025-05-06T15:12:35.6697380Z     bp::child c("dummy", bp::std_out > stdout_stream, bp::std_err > stdout_stream, bp::std_in < stdin_stream);
2025-05-06T15:12:35.6697480Z              ^
2025-05-06T15:12:35.6697570Z              ;
2025-05-06T15:12:35.6697900Z conftest.cpp:84:9: error: no member named 'child' in namespace 'boost::process'
2025-05-06T15:12:35.6698310Z     bp::child c("dummy", bp::std_out > stdout_stream, bp::std_err > stdout_stream, bp::std_in < stdin_stream);
2025-05-06T15:12:35.6698400Z     ~~~~^
2025-05-06T15:12:35.6698730Z conftest.cpp:84:88: error: no member named 'std_in' in namespace 'boost::process'
2025-05-06T15:12:35.6699150Z     bp::child c("dummy", bp::std_out > stdout_stream, bp::std_err > stdout_stream, bp::std_in < stdin_stream);
2025-05-06T15:12:35.6699270Z                                                                                    ~~~~^
2025-05-06T15:12:35.6699610Z conftest.cpp:84:30: error: no member named 'std_out' in namespace 'boost::process'
2025-05-06T15:12:35.6700020Z     bp::child c("dummy", bp::std_out > stdout_stream, bp::std_err > stdout_stream, bp::std_in < stdin_stream);
2025-05-06T15:12:35.6700120Z                          ~~~~^
2025-05-06T15:12:35.6700410Z conftest.cpp:84:40: error: use of undeclared identifier 'stdout_stream'
2025-05-06T15:12:35.6700820Z     bp::child c("dummy", bp::std_out > stdout_stream, bp::std_err > stdout_stream, bp::std_in < stdin_stream);
2025-05-06T15:12:35.6700930Z                                        ^
2025-05-06T15:12:35.6701260Z conftest.cpp:84:59: error: no member named 'std_err' in namespace 'boost::process'
2025-05-06T15:12:35.6702110Z     bp::child c("dummy", bp::std_out > stdout_stream, bp::std_err > stdout_stream, bp::std_in < stdin_stream);
2025-05-06T15:12:35.6702230Z                                                       ~~~~^
2025-05-06T15:12:35.6702510Z conftest.cpp:84:69: error: use of undeclared identifier 'stdout_stream'
2025-05-06T15:12:35.6702920Z     bp::child c("dummy", bp::std_out > stdout_stream, bp::std_err > stdout_stream, bp::std_in < stdin_stream);
2025-05-06T15:12:35.6703310Z                                                                     ^
2025-05-06T15:12:35.6703720Z conftest.cpp:86:9: error: use of undeclared identifier 'c'
2025-05-06T15:12:35.6703990Z     if (c.running()) c.terminate();
2025-05-06T15:12:35.6704080Z         ^
2025-05-06T15:12:35.6704310Z conftest.cpp:86:22: error: use of undeclared identifier 'c'
2025-05-06T15:12:35.6704430Z     if (c.running()) c.terminate();
2025-05-06T15:12:35.6704530Z                      ^
2025-05-06T15:12:35.6704750Z conftest.cpp:87:5: error: use of undeclared identifier 'c'
2025-05-06T15:12:35.6705050Z     c.wait();
2025-05-06T15:12:35.6705150Z     ^
2025-05-06T15:12:35.6705490Z conftest.cpp:88:5: error: use of undeclared identifier 'c'
2025-05-06T15:12:35.6705610Z     c.exit_code();
2025-05-06T15:12:35.6705700Z     ^
2025-05-06T15:12:35.6705960Z 15 errors generated.

So disable the external signer to avoid CI failures.

[hebasto]

Going to merge this PR to unlock the CI.

[hebasto]

Additionally, see boostorg/process#480.