[vcpkg-tool] vcpkg fetch shouldn't require a setup registry cache

[Neumann-A]

Repo: Unset LOCALAPPDATA in your local environment

(vcpkg) PS D:\vcpkg_folders\no_msvc_project> .\vcpkg\vcpkg.exe fetch cmake
both %LOCALAPPDATA% and %APPDATA% were unreadable
(vcpkg) PS D:\vcpkg_folders\no_msvc_project> $Env:VCPKG_DOWNLOADS
E:\vcpkg_cache\downloads

It works after setting up X_VCPKG_REGISTRIES_CACHE

(vcpkg) PS D:\vcpkg_folders\no_msvc_project> .\vcpkg\vcpkg.exe fetch ninja
Value of environment variable X_VCPKG_REGISTRIES_CACHE is not a directory: E:\vcpkg_cache\registries
(vcpkg) PS D:\vcpkg_folders\no_msvc_project> .\vcpkg\vcpkg.exe fetch ninja
E:\vcpkg_cache\downloads\tools\ninja-1.11.1-windows\ninja.exe

but there is no reason for vcpkg to look up that path since "fetch" doesn't use it.

[Mengna-Li]

According to my understanding, the fetch command first goes to the vcpkg download path to search for the package. If it cannot be found, it will be downloaded and placed in the download path. Why search for the X_VCPKG_REGISTRIES_CACHE. @vicroms Can you help explain it? I couldn't find any relevant explanation.