Fix to commit 893a06e062 which broke the SIGMA_SHELF_ZSTAR option

[MJHarrison-GFDL]

• Testing with an ice shelf (e.g. ISOMIP) and using
  the SIGMA_SHELF_ZSTAR coordinate would have caught this bug.

[codecov]

Codecov Report

Merging #1459 (360eb2f) into dev/gfdl (3a0725d) will not change coverage.
The diff coverage is 0.00%.

❗ Current head 360eb2f differs from pull request most recent head bf34a57. Consider uploading reports for the commit bf34a57 to get more accurate results

@@            Coverage Diff            @@
##           dev/gfdl    #1459   +/-   ##
=========================================
  Coverage     29.30%   29.30%           
=========================================
  Files           234      234           
  Lines         70600    70600           
=========================================
  Hits          20686    20686           
  Misses        49914    49914           

Impacted Files	Coverage Δ
src/ALE/MOM_regridding.F90	21.36% <0.00%> (ø)

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[marshallward]

https://gitlab.gfdl.noaa.gov/ogrp/MOM6/-/pipelines/13301 ✔️