Use nightly Dask in GPU CI testing

[charlesbluca]

@jakirkham suggested potentially testing against nightly Dask in this repo's GPU CI (#318 (comment)); this could be nice to get a sense of when breaking changes are coming, but also contrasts the CPU CI matrix, which tests against pinned Dask versions.

cc @GenevieveBuckley do you have any thoughts here?

[GenevieveBuckley]

I'm not sure what actionable information this would give us, really. Maintainer capacity on this repo is pretty low.

In the event the nightly CI did find a problem, it seems most probable that we'd just file an issue in dask/dask (and maybe we'd plan on looking into it properly at some stage, but the main dask repo is more active and probably someone else would get to it first). I also don't think we're particularly useful as an early warning system for dask, because activity over here is pretty infrequent.

All in all I'm not opposed, just not sure that having the extra information will cause us to do anything differently.