Update GH Actions to use mamba

[tomvothecoder]

Description

• Closes [DevOps]: Switch GitHub Actions build job to use mamba instead of conda #449

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules

If applicable:

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass with my changes (locally and CI/CD build)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have noted that this is a breaking change for a major release (fix or feature that would cause existing functionality to not work as expected)

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (c9bcbcd) 100.00% compared to head (a81973d) 100.00%.

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Additional details and impacted files

@@            Coverage Diff            @@
##              main      #450   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           14        14           
  Lines         1308      1308           
=========================================
  Hits          1308      1308           

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[tomvothecoder]

Results: This effectively decreases the total build time from 1-2 minutes depending on the Python version.

Python 3.9 build with conda takes about 4.5 mins: https://github.com/xCDAT/xcdat/actions/runs/4683848502/jobs/8299360740
Python 3.9 build with mamba takes about 2.5 mins: https://github.com/xCDAT/xcdat/actions/runs/4704284564/jobs/8343771268