Optimize JsonUpdateRepository

[fthomas]

This should save a few cycles and reduce disk I/O.

[codecov]

Codecov Report

Merging #640 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master     #640   +/-   ##
=======================================
  Coverage   60.25%   60.25%           
=======================================
  Files          66       66           
  Lines         878      878           
  Branches       17       17           
=======================================
  Hits          529      529           
  Misses        349      349

Impacted Files	Coverage Δ
...rg/scalasteward/core/update/json/UpdateStore.scala	0% <ø> (ø)	⬆️
...a/org/scalasteward/core/update/UpdateService.scala	0% <0%> (ø)	⬆️
...teward/core/update/json/JsonUpdateRepository.scala	0% <0%> (ø)	⬆️

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