Issue #748: adding a fallback for file sinks

[grantlouisherman]

Issue number of the reported bug or feature request: #748

Describe your changes
I have added a backup for creating files when mmap or other linux based kernel functions are not available

Testing performed
Added two unit tests to test to make sure that if the env variable is set then we fallback to using just ::write

[codecov-commenter]

Codecov Report

❌ Patch coverage is 89.41176% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.37%. Comparing base (5b58e31) to head (542adb0).

Files with missing lines	Patch %	Lines
src/memray/_memray/sink.cpp	73.52%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #945      +/-   ##
==========================================
- Coverage   92.45%   92.37%   -0.09%     
==========================================
  Files          99       99              
  Lines       11787    11865      +78     
  Branches      430      430              
==========================================
+ Hits        10898    10960      +62     
- Misses        889      905      +16     

Flag	Coverage Δ
cpp	92.37% <89.41%> (-0.09%)	⬇️
python_and_cython	92.37% <89.41%> (-0.09%)	⬇️

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