Revert "Skip for-loop alloc in go/vt/discovery/healthcheck.go"

[dbussink]

Reverts #15326

I don't think this actually changes any allocation. It moves the definition, but the statement itself does not allocate (it's a nil slice).

Moving it means we could maybe accidentally reuse it later on from a for loop, I think within a loop it's best to keep the declaration as close to the usage as possible.

[vitess-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e549980) 67.63% compared to head (64498ce) 67.71%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15328      +/-   ##
==========================================
+ Coverage   67.63%   67.71%   +0.08%     
==========================================
  Files        1561     1561              
  Lines      193392   194013     +621     
==========================================
+ Hits       130794   131383     +589     
- Misses      62598    62630      +32     

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