Log Message:
obconf-qt, libfm-qt, qterminal: Nix explicit CMAKE_PREFIX_PATH.

CMAKE_PREFIX_PATH is already added to CMAKE_CONFIGURE_ARGS by
devel/cmake/build.mk, and ${PREFIX}/qt6 is already included in it by
x11/qt6-qtbase/buildlink3.mk.

Revbump since this could in principle change the package, if the
pkgsrc variable CMAKE_PREFIX_PATH had anything else in it which is
now no longer hidden.