Fix temporary file name collisions in hictk convert *cool -> hic

[robomics]

This PR fixes the bug reported in #271.

This bug is a regression introduced by #210.

In brief, when performing *cool -> hic conversion, a randomly-named, self-deleting temporary directory was created under the default temporary directory (e.g. /tmp) or a user provided temporary folder.
However, due to a silly mistake, the parent of this folder was passed to one of the functions creating temporary files. When converting multiple files sharing one or more resolution in parallel using the default temporary folder, this results in unintended file truncation (which could lead to all kinds of errors).

Other commands creating .hic files (e.g. hictk load and hictk zoomify) are not affected.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.01%. Comparing base (0dafc0d) to head (a59f894).
Report is 102 commits behind head on main.

Files with missing lines	Patch %	Lines
src/hictk/convert/cool_to_hic.cpp	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #274      +/-   ##
==========================================
- Coverage   79.02%   79.01%   -0.02%     
==========================================
  Files         171      171              
  Lines       16324    16327       +3     
  Branches     2250     2250              
==========================================
  Hits        12900    12900              
- Misses       2455     2457       +2     
- Partials      969      970       +1     

Flag	Coverage Δ
[tests	integration](https://app.codecov.io/gh/paulsengroup/hictk/pull/274/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=paulsengroup)	69.38% <80.00%> (-0.04%)
[tests	unittests](https://app.codecov.io/gh/paulsengroup/hictk/pull/274/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=paulsengroup)	76.43% <100.00%> (-0.01%)

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