cmState: Skip variable scope snapshots to avoid call stack duplicates
Since commit v3.4.0-rc1~179^2~1 (cmState: Add a VariableScope snapshot type, 2015-08-23) the snapshot stack may have a VariableScopeType entry. Skip over these when constructing the call stack, just as we do for policy scopes. Otherwise we report the command causing the variable scope to be entered twice (e.g. find_package while loading a package version file).