SSH private key secret has changed its name

[kelson42]

• Change the secret name to use new secret
• Improve the naming of a few variables

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.59%. Comparing base (3b1cce6) to head (eeb4f00).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1423   +/-   ##
=======================================
  Coverage   92.59%   92.59%           
=======================================
  Files          18       18           
  Lines         878      878           
=======================================
  Hits          813      813           
  Misses         65       65           

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[BPerlakiH]

@kelson42 We have the same variables on the custom-apps side as well, although those seems not to be in use at the moment (I suspect mainly due to the fact that we do not upload DMGs for custom apps).
Maybe we should update them as well, for consistency:
https://github.com/kiwix/kiwix-apple-custom/blob/4168c85ddf3444b0bcd4d8d7c6774575bf0ae9b1/.github/workflows/cd.yml#L13
https://github.com/kiwix/kiwix-apple-custom/blob/4168c85ddf3444b0bcd4d8d7c6774575bf0ae9b1/.github/workflows/ci.yml#L16