[vcpkg] Allow compilation of HEAD_REF-only ports in manifest mode

[PazerOP]

Requires microsoft/vcpkg-tool#24 to work, otherwise it does nothing.

Sort of addresses #12504. More specifically, supports my desired use case:

• I have a registry with very, very frequently touched package. It does not make sense for me to version this package in any way, I am the only one using it, and I only ever want to use the latest version. Therefore, I have it available on my vcpkg registry as a HEAD_REF-only port.
• I have a project that consumes previously mentioned package, as well as many others from the official vcpkg registry. I do not want --head behavior for all those packages.
• I want to use manifest mode, which means I cannot currently specify per-port build options (here's where [vcpkg|manifest] Missing a way to pass --head to certain dependencies.  #12504 comes into play)

With the given conditions, it is impossible to build my project. vcpkg will run the manifest install step, then hit my --head-requiring port, and abort.

I cannot use --head, because not all my vcpkg dependencies are built from source (discord-game-sdk). I also have no desire to be living on the bleeding edge for any third-party dependencies anyway.

Theoretically, this change should not break any existing behavior (unless people were relying on vcpkg not working in this situation). It takes a previously non-working scenario (HEAD_REF-only port, manifests, and no --head) and makes it work in a (in my opinion) reasonable way.

• What does your PR fix? Somewhat related to [vcpkg|manifest] Missing a way to pass --head to certain dependencies.  #12504

• Which triplets are supported/not supported? Have you updated the CI baseline?
  I do not believe this has any affect for CI pipelines, but I have not studied them in detail. Please correct me if this is wrong.

• Does your PR follow the maintainer guide?
  To the best of my understanding, yes.

[ras0219-msft]

We are very hesistant to introduce any functionality that produces nondeterministic behavior, especially without loud, obnoxious messages for/from users.

Have you considered just defining VCPKG_USE_HEAD_VERSION yourself in your port?

set(VCPKG_USE_HEAD_VERSION ON)

Note: this is not a supported scenario and breaks binarycaching, but it is a hack that may give you what you want.

[ras0219-msft]

Closing for now, let me know if the above solution doesn't work for you!