Fix saving relative symlink for ModelCheckpoint callback

[shenmishajing]

What does this PR do?

Fixes #19294

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📚 Documentation preview 📚: https://pytorch-lightning--19303.org.readthedocs.build/en/19303/

[awaelchli]

Thank you for sending the PR! Great work. I added the test as we discussed.

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[codecov]

Codecov Report

Merging #19303 (d414025) into master (e89f46a) will decrease coverage by 35%.
The diff coverage is 100%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #19303      +/-   ##
==========================================
- Coverage      84%      49%     -35%     
==========================================
  Files         446      438       -8     
  Lines       37570    37417     -153     
==========================================
- Hits        31383    18291   -13092     
- Misses       6187    19126   +12939