Spack + Dakota 6.18

[tdrayna]

I was able to successfully update spack to build Dakota 6.18 using the public GitHub repository. If anyone is interested or can review my PR please see this:

PR#39882

Thanks!
Travis

[jadamstephens]

Travis,

Thank you! We are very novice spack users, so I doubt my review would be useful. I will say that nothing looks obviously suspicious to me in the changeset. We definitely appreciate you making this available.

Adam