Change artifact path for heat-diffusion on buildkite

[imreddyTeja]

Purpose

Save the output plots of the heat-diffusion experiment on buildkite.
closes #1095

To-do

Content

• changed artifact path to match output path
• Now build kite heat-diffusion shows 3 saved artifacts vs 0 before

---

• I have read and checked the items on the review checklist.

[juliasloan25]

Good catch! I missed the heat diffusion example when I changed the artifact paths in e4bbc30. I changed the paths so the artifacts are in a subdirectory of the directory created for the job, rather than another directory at the same level (e.g. amip_default_artifacts/ became amip_default/artifacts/. To keep the heat diffusion example consistent with everything else, we should update the dir used in the example (here), rather than in the pipeline.