build: disable external-signer for Windows

It's come to light that Boost ASIO (a Boost Process sub dep) has in some
instances, been queitly initialising our network stack on Windows (see
PR #28486 and discussion in #28940).

This has been shielding a bug in our own code, but the larger issue
is that Boost Process/ASIO is running code before main, and doing things
like setting up networking. This undermines our own assumptions about
how our binary works, happens before we get to run any sanity checks,
and also runs before we call our own code to setup networking.

It's also not clear why a feature like external signer would have a
dependency that would be doing anything network/socket related, given it
only exists to spawn a local process.

build_msvc/vcpkg.json

@@ -5,7 +5,6 @@
     "berkeleydb",
     "boost-date-time",
     "boost-multi-index",
-    "boost-process",
     "boost-signals2",
     "boost-test",
     "libevent",