Update Icepack, minor fixes from prerelease testing

[apcraig]

PR checklist

• Short (1 sentence) summary of your PR:
  Update Icepack
• Developer(s):
  apcraig
• Suggest PR reviewers from list in the column to the right.
• Please copy the PR test results link or provide a summary of testing completed below.
  Tested mostly on conda. Changes are highly limited. Will undergo release testing once merged.
• How much do the PR code changes differ from the unmodified code?
  • bit for bit
  • different at roundoff level
  • more substantial
• Does this PR create or have dependencies on Icepack or any other models?
  • Yes
  • No
• Does this PR add any new test cases?
  • Yes
  • No
• Is the documentation being updated? ("Documentation" includes information on the wiki or in the .rst files from doc/source/, which are used to create the online technical docs at https://readthedocs.org/projects/cice-consortium-cice/. A test build of the technical docs will be performed as part of the PR testing.)
  • Yes
  • No, does the documentation need to be updated at a later time?
    • Yes
    • No
• Please provide any additional information or relevant details below:

Fix error in gx1coreii initial condition filename
Update conda macos Macros
Add documentation about updating the conda environment

[phil-blain]

I'm a little curious about why this is suddenly needed, but I guess something changed in the conda-forge toolchain...

[apcraig]

I don't know. This is what I had to do on my new mac running 11.2.1. I hadn't been able to run the conda macos before on this machine (although I had quickly tried before). I also had to make sure I updated my conda environment because I had an older install that had the omp bug. The extra LD could be a bug in the underlying conda setup, but I personally don't really know enough to be sure.