(Manifest mode) Defer port build until the build step

[ddavness]

Right now and when in manifest mode, at the configuration step, vcpkg goes ahead and builds all the necessary dependencies for you - but in some cases it is not very convenient.

My project has a CI job that just runs clang-format and clang-tidy on the whole codebase to confirm the code is up to standard - in particular clang-tidy obviously needs the libraries' headers. The issue is that to just to get said headers, I need to build all the dependencies (including ICU which takes a while, and for some reason I can't get it into the GitHub Registry to cache either), which in this case is absolutely unnecessary.

Is there a way to defer the building of the dependencies until the build step? (i.e. cmake --build build)