Make AUDITWHEEL_LD_LIBRARY_PATH override DT_RUNPATH

[1998LJ]

Closes #737.

AUDITWHEEL_LD_LIBRARY_PATH currently shares the same internal search-path bucket as LD_LIBRARY_PATH, which places both after an ELF's DT_RUNPATH.

This separates the auditwheel-specific environment path from the standard LD_LIBRARY_PATH and gives it the lower-risk precedence discussed in #737:

RPATH > AUDITWHEEL_LD_LIBRARY_PATH > RUNPATH > LD_LIBRARY_PATH

The existing precedence of RUNPATH over the standard LD_LIBRARY_PATH is intentionally preserved.

The change also keeps compatibility with callers that provide the existing ldpaths dictionary shape without the new auditwheel-specific entry.

Tests cover:

• RUNPATH continuing to override LD_LIBRARY_PATH;
• AUDITWHEEL_LD_LIBRARY_PATH overriding RUNPATH;
• all three paths being present simultaneously;
• root != "/" behavior;
• legacy ldpaths dictionaries.

Validation:

• pytest tests/unit/test_lddtree.py
• pytest tests/unit
• nox -s lint
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.22%. Comparing base (b9a93b0) to head (405980f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #738      +/-   ##
==========================================
+ Coverage   96.07%   96.22%   +0.15%     
==========================================
  Files          23       23              
  Lines        2036     2038       +2     
  Branches      388      389       +1     
==========================================
+ Hits         1956     1961       +5     
+ Misses         46       44       -2     
+ Partials       34       33       -1     

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