Protect NetCDF saving from bad Python-vs-HDF file lock timing

[trexfeathers]

🚀 Pull Request

Description

Thanks to @RachelNorth and @TeresaHughes for reporting this.

Even when our NetCDF save operations are fully serialised - no parallelism - HDF still occasionally fails to acquire the file. This is despite all Python locks being available at expected moments, and the file reporting as closed. During testing, 2nd retry always succeeded. This is likely caused by HDF-level locking running on a different timescale to Python-level locking - i.e. sometimes Python has released its locks but HDF still has not. Thought to be filesystem-dependent; further investigation is needed but time is limited at the moment so it seemed best to just get the protective code in immediately.

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Codecov Report

❌ Patch coverage is 66.66667% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.28%. Comparing base (24e258f) to head (380d0eb).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/iris/fileformats/netcdf/_thread_safe_nc.py	66.66%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6760      +/-   ##
==========================================
+ Coverage   90.25%   90.28%   +0.03%     
==========================================
  Files          91       91              
  Lines       24630    24664      +34     
  Branches     4609     4620      +11     
==========================================
+ Hits        22229    22269      +40     
+ Misses       1624     1621       -3     
+ Partials      777      774       -3     

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