ci: Disable external signer in native macOS CI job

Boost.Process 1.88 currently fails the configure test, so disable the
external signer to avoid CI failures.

Author: hebasto

ci/test/00_setup_env_mac_native.sh

@@ -9,7 +9,7 @@ export LC_ALL=C.UTF-8
 export HOST=x86_64-apple-darwin
 export PIP_PACKAGES="zmq"
 export GOAL="install"
-export BITCOIN_CONFIG="--with-gui --with-miniupnpc --with-natpmp --enable-reduce-exports"
+export BITCOIN_CONFIG="--with-gui --with-miniupnpc --with-natpmp --enable-reduce-exports --disable-external-signer"
 export CI_OS_NAME="macos"
 export NO_DEPENDS=1
 export OSX_SDK=""