MAINT: A lockfile is needed

Since there seems to be a version bump on `xtensor` which breaks `flowy` builds, https://dev.azure.com/conda-forge/feedstock-builds/_build/results?buildId=1303232&view=logs&j=dff6ff7b-316b-540a-5923-739df25f7012&t=2582809e-c3dc-5a5f-d883-fd3c8f9139aa

`xbuilder` is now in `generators/xbuilder`.