doc: build: Fix instructions for msvc gui builds #31851

If the instructions in doc/build-windows-msvc.md are followed as-is, and "Developer (PowerShell|Command Prompt) for VS 2022" is used to execute the suggested build commands, the root directory of vcpkg (e.g. in VS 2022 Community edition: C:\Program Files\Microsoft Visual Studio\2022\Community\VC\vcpkg), is too long, and when vcpkg attempts to build any of the QT packages, it will fail because of build steps that require path lengths greater than Windows' MAX_PATH 260 character limit. This can be avoided without needing to move the vcpkg root dir by setting --x-buildtrees-root to a short path, like C:\vcpkg.

See e.g. https://github.com/microsoft/vcpkg/issues/28451, https://github.com/microsoft/vcpkg/issues/28083, https://github.com/microsoft/vcpkg/issues/24751.

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For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/31851.

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My only concern is that --x-buildtrees-root=<path> is labeled as:

an experimental feature of vcpkg which may change or be removed at any time.

I considered that, but this solution is much more sane than any of the others I managed to come up with, and the flag has been around for at least 4 years (https://github.com/microsoft/vcpkg/issues/13789#issuecomment-700140607) , and this is only documentation, if there is a better solution, especially one that doesn't require the user to intervene in any way, that would be ideal.

ACK c3fa043ae560289759b6f836ac62736d97ba91bf.

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Am I misremembering or was this, or a similar workaround, part of the previous build instructions at some point?

ACK c3fa043

Am I misremembering or was this, or a similar workaround, part of the previous build instructions at some point?

Yep, #29048 added a similar note to the old build_msvc/README.md, which had instructions for doing a static build of Qt, but I guess during the cmake transition, there was a switch from manually building Qt to using vcpkg, but sadly, the same problem still exists.

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