Thanks @ben.boeckel for the suggestions. In the GA runner, the exhaustive list doesn’t contain my path, which is easy to spot, because it’s on D:\ whereas all the system paths listed are on C:\. They seem like to be related to PATH , as every entry is a pre-installed tool.
I’m not using Vcpkg at all (that entry may be misleading), I didn’t provide the toolchain file on the command-line so it better not interfere with anything.
Can environment variables alter the behavior of CMAKE_PREFIX_PATH? Can any override it?