Save index file in postgres data dir

[var77]

There was an issue in some environments that due to limited size of /tmp directory the process was failing to save large index files there.

I have changed the temporary data directory for lantern indexes to be $pg_data_dir/ldb-index-$relfilenode to avoid that limitation.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (0d14aa4) 81.24% compared to head (50935fc) 81.25%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #256      +/-   ##
==========================================
+ Coverage   81.24%   81.25%   +0.01%     
==========================================
  Files          17       17              
  Lines        1477     1478       +1     
  Branches      338      338              
==========================================
+ Hits         1200     1201       +1     
  Misses        126      126              
  Partials      151      151              

Files	Coverage Δ
src/hnsw/build.c	79.68% <100.00%> (+0.08%)	⬆️