Use ports-root and registry-versions git repos.

[BillyONeal]

Use the git repo of --x-builtin-ports-root and --x-builtin-registry-versions-dir when talking about port commit shas and version tree shas, respectively.

I believe this is necessary to make the instructions in https://learn.microsoft.com/vcpkg/produce/publish-to-a-git-registry#4---update-the-versions-database actually work.

This also reduces the number of things that assume VCPKG_ROOT is a git repo. (Since it is not when we are shipped in VS or acquired with the one liner)

Another extraction from #1210 .

[vicroms]

The current instructions worked for x-add-version because the calculated SHAs are the same regardless of the parent repository. But this change could let us to run x-ci-verify-versions on the registry.

[BillyONeal]

The current instructions worked for x-add-version because the calculated SHAs are the same regardless of the parent repository.

x-add-version depends on ls-tree doing the right thing, and I don't believe it does so in the wrong repo. It certainly doesn't do the right thing for one-liner and VS copy of vcpkg, where vcpkg_root isn't a git repo at all.

But this change could let us to run x-ci-verify-versions on the registry.

Indeed this is why I made the change in #1210 :)