fix: simd in immutable::nearest_one

[sdd]

I have no idea why these tests work locally but fail on GitHub Actions. They also pass if I run them on a t2.micro (has AVX2 but not AVX512) or on a c5.large (has AVX2 and AVX512) instance on AWS EC2.

Fixed. I was setting the RUSTFLAGS env var on the wrong build step 🤦🏼

[codecov-commenter]

Codecov Report

Merging #97 (ee120f6) into master (98c907b) will increase coverage by 0.89%. The diff coverage is 95.23%.

@@            Coverage Diff             @@
##           master      #97      +/-   ##
==========================================
+ Coverage   90.34%   91.23%   +0.89%     
==========================================
  Files          41       41              
  Lines        5374     5455      +81     
  Branches     5374     5455      +81     
==========================================
+ Hits         4855     4977     +122     
+ Misses        488      449      -39     
+ Partials       31       29       -2     

Files	Coverage Δ
src/float_leaf_simd/leaf_node.rs	67.85% <66.66%> (+18.52%)	:arrow_up:
src/immutable/float/query/nearest_one.rs	90.06% <97.43%> (+9.16%)	:arrow_up:

... and 3 files with indirect coverage changes

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