Contribute Spack package for XIOS3

[joewallwork]

There is now a Spack package for XIOS
https://packages.spack.io/package.html?name=xios
Using this would help fix a few difficulties, e.g., the fact that the Docker CI sometimes fails to checkout XIOS. However, only 2.6 is included, not version 3. Perhaps we could contribute an update to install version 3?

[joewallwork]

Hopefully it would be as simple as adding another version like https://github.com/spack/spack-packages/blob/8913ea5052d65511819246a6038e4081e133801d/repos/spack_repo/builtin/packages/xios/package.py#L17-L21.

[joewallwork]

I drafted something but there isn't currently a stable release for XIOS3 so we should probably wait until then to open the PR to Spack-packages.