CI: unset VCPKG_ROOT

[MehdiChinoune]

Recently GHA started defining VCPKG_ROOT environment variable (actions/runner-images#6192).
So unset it to not let it autodetected and interfere with our build jobs.

[Biswa96]

The build log of qt6-doc has many vcpkg related variables. Should not all of the be unset? 💭

[lazka]

Just curious, is there some package that failed because of this?

[MehdiChinoune]

Just curious, is there some package that failed because of this?

I think PkgConfig couldn't be found because of it (#13367).
vcpkg has its own CMAKE_TOOLCHAIN_FILE that defines many CMake variables and functions, which could interfere with our default CMake settings.

[lazka]

I see, thanks

[MehdiChinoune]

The build log of qt6-doc has many vcpkg related variables. Should not all of the be unset? 💭

The main reason is that vcpkg get autodetected in https://invent.kde.org/qt/qt/qtbase/-/blob/6.4.0/cmake/QtAutoDetect.cmake#L533 which rely on the environment variable VCPKG_ROOT to be defined in https://invent.kde.org/qt/qt/qtbase/-/blob/6.4.0/cmake/QtAutoDetect.cmake#L159

[lazka]

Also done in autobuild: msys2/msys2-autobuild@c9dd9af

[MehdiChinoune]

actions/runner-images#6376