use conda-forge's cling?

So my understanding here is a bit hazy, but AFAIU root and cling are developed in close collaboration, and root vendors cling at least partially, c.f. log snippets of the kind
```
Making $SRC_DIR/build-rootcling_stage1-xp/ginclude/compiledata.h
```
or
```
-- Enabled support for: builtin_cling rpath shared
```

Now that conda-forge has cling builds [again](https://github.com/conda-forge/cling-feedstock/pull/62), it might be worthwhile to reuse that.

Not sure how easy or practical that would be (given that root and cling seem to have similar but _different_ patch sets on top of llvmdev & clangdev), but it seems worth discussing at least.

CC @conda-forge/root @conda-forge/cling