Merge bitcoin/bitcoin#24065: build: explicitly disable support for external signing on Windows

e2ab9f83f8655bf09ea392beeee36b2bbe29769b build: disable external signer on Windows (fanquake)

Pull request description:

  This change explicitly disables support for external signing when targeting Windows and OpenBSD. The driver for this is that Boost Process uses boost::filesystem internally, when targeting Windows, which gets in the way of removing our usage of it (#20744). While we could adjust #20744 to still link against the Boost libs when building for Windows, that would be disappointing, as we wouldn't have cleanly removed the Boost usage we're trying too (including the build infrastructure), and, we'd be in a position where we would be building releases differently depending on the platform, which is something I want to avoid.

  After discussion with Sjors, Achow and Hebasto, this seemed like a reasonable step to move #20744 forward (as-is). Note that support for external signing ([while already being experimental](https://github.com/bitcoin/bitcoin/blob/master/doc/external-signer.md#example-usage)), could be considered even more experimental on Windows. Also, oddly, we have external-signing [explicitly disabled in our Windows (cross-compile) CI](https://github.com/bitcoin/bitcoin/blob/807169e10b4a18324356ed6ee4d69587b96a7c70/ci/test/00_setup_env_win64.sh#L16), it's not clear why this is the case, as, if it's a feature being built into releases, it should be being built and tested in the CI which is most-like the release process.

  There is an [issue open upstream](https://github.com/boostorg/process/issues/207), in regards to migrating Boost Process to std::filesystem, or having an option to use it. However there hasn't been much discussion since it was opened ~9 months ago. There is another related issue here: https://github.com/klemens-morgenstern/boost-process/issues/164.

  Resolves #24036.

ACKs for top commit:
  Sjors:
    utACK e2ab9f8
  achow101:
    ACK e2ab9f83f8655bf09ea392beeee36b2bbe29769b
  kallewoof:
    utACK e2ab9f83f8655bf09ea392beeee36b2bbe29769b
  hebasto:
    ACK e2ab9f83f8655bf09ea392beeee36b2bbe29769b, tested on Linux Mint 20.2 (x86_64).

Tree-SHA512: 36fcfc0e1a008a8271dc76b8e12e93d3e1d1e528bf668e95a559e9f6fd7d5f031bd7a6a6bc8b9fa9d057b2cd56f9ec8838c7f74e87899bf9a6aeb787afbd112c

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