308aec3 build: disable external-signer for Windows (fanquake)
3553731 ci: remove --enable-external-signer from win64 job (fanquake)

Pull request description:

  It's come to light that Boost ASIO (a Boost Process sub dep) has in some
  instances, been quietly  initialising our network stack on Windows (see
  PR bitcoin/bitcoin#28486 and discussion in bitcoin/bitcoin#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 run any sanity checks,
  and before we call our own code to setup networking. Note that ASIO also
  calls WSAStartup with version `2.0`, whereas we call with `2.2`.

  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.

  See also the discussion in bitcoin/bitcoin#24907. Note that the maintaince of Boost Process in general,
  has not really improved. For example, rather than fixing bugs like boostorg/process#111,
  i.e, boostorg/process#317, the maintainer chooses to just wrap exception causing overflows
  in try-catch blocks: boostorg/process@0c42a58. These changes get merged in large,
  unreviewed PRs, i.e boostorg/process#319.

  This PR disables external-signer on Windows for now. If, in future, someone
  changes how Boost Process works, or replaces it entirely with some
  properly reviewed and maintained code, we could reenable this feature on
  Windows.

ACKs for top commit:
  hebasto:
    re-ACK 308aec3.
  TheCharlatan:
    ACK 308aec3

Tree-SHA512: 7405f7fc9833eeaacd6836c4e5b1c1a7845a40c1fdd55c1060152f8d8189e4777464fde650e11eb1539556a75dddf49667105987078b1457493ee772945da66e