feat(tempfile): allow usage of custom tempdir locations

[n0toose]

This is intended to let users optionally use custom tmpfs filesystems or store unmapped files in directories that are not monitored by the temporary file cleaner.

[codecov]

Codecov Report

Attention: Patch coverage is 67.85714% with 9 lines in your changes missing coverage. Please review.

Project coverage is 71.69%. Comparing base (da4b63e) to head (5a66a38).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/isolation/tempdir.rs	61.11%	7 Missing ⚠️
src/bin/uhyve.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #856      +/-   ##
==========================================
- Coverage   71.86%   71.69%   -0.18%     
==========================================
  Files          23       23              
  Lines        3064     3077      +13     
==========================================
+ Hits         2202     2206       +4     
- Misses        862      871       +9     

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[jounathaen]

LGTM! Thank you :-)