Fix resolution of weights filename in TMVATransform

Before this fix, the filename of the weights in TMVATransform was sometimes resolved (env vars expanded) but sometimes it wasn't, notably when passing it to ROOT.

This bug prevents building the functor cache in Moore when TMVATransform is used.

The positive effect of this bug fix is not observable in the nightlies, so we should probably pair it with a MR in Moore.

Thanks to @sstahl for reporting