refactor: use new versions of environment variables

[romanc]

Description

The old ones are currently deprecated and expected to be removed in the future (after a grace period). See
NOAA-GFDL/NDSL#193 for more details.

How Has This Been Tested?

All good as CI is still green.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas: N/A
• I have made corresponding changes to the documentation: N/A
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included: N/A
• Targeted model, if this change was triggered by a model need/shortcoming: N/A

[romanc]

/cc @fmalatino for review

[fmalatino]

Tests will pass post-merge of PR 60

[oelbert]

Good to go