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).