Pin tblite to v0.6.0 in Findtblite.cmake (CMake)

## Summary

CMake builds of xtb still fetch tblite at `HEAD`, while Meson is already pinned to `v0.6.0`. Please pin `cmake/modules/Findtblite.cmake` to `v0.6.0` as well so both build systems are consistent and reproducible.

## Related

- #1408 pinned Meson only (`subprojects/tblite.wrap` → `revision = v0.6.0`)
- `cmake/modules/Findtblite.cmake` still has:

```cmake
set(_rev "HEAD")