Help mamba decide on CUDA variant

Mamba currently can't properly decide which `tensorflow` cuda package to prefer since we don't directly depend on `cudatoolkit` in the `tensorflow` package.

Only in the `tensorflow-base` package we have a `cudatoolkit` dependency.

However, `tensorflow-base` is connected to `tensorflow` via an exact dependency.

If we'd also add the `cudatoolkit` to the `tensorflow` package, mamba would be able to select the best version.

OR we could remove the tensorflow variant completely and just have a single metapackage that depends on `tensorflow-base {{ VERSION }}` (but not exact). Then mamba could decide the tensorflow-base version more freely.