Respect VCPKG_ROOT by... ignoring it? 🤯

[keynmol]

About 20 days ago, GH runners started settings VCPKG_ROOT variable:
actions/runner-images#6228

This causes basically all of my sbt-vcpkg-based projects to fail.

Through vigorous scientific testing in this PR I've determined that it's indeed caused by the variable and it can be overridden if necessary.

I think the plan should be:

1. Harden the setting for VcpkgRoot
   • Users should be able to set them manually
   • Users should specify to use the environment variable
   • Users should be able to rely on default cache locations (current behaviour)
   • Users should specify whether they want to allow bootstrapping
   • All the different init mechanisms should come with a orElse method to build a fallback chain
2. Set the default mode to VcpkgRoot.FromEnv(allowBootstrap = true).orElse(VcpkgRoot.SystemCache(allowBootstrap = true) - "if the env variable is present, use that, and if the vcpkg binary is not there - allow bootstrapping that location. if the variable is not present, use system cache location"