openapi: Avoid recursive cache update to support future Scala 2.13.x

[xerial]

A fix for scala/community-build#1568

[codecov]

Codecov Report

Merging #2269 (aebd041) into master (55ba110) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2269      +/-   ##
==========================================
+ Coverage   82.57%   82.58%   +0.01%     
==========================================
  Files         315      315              
  Lines       12442    12444       +2     
  Branches      787      785       -2     
==========================================
+ Hits        10274    10277       +3     
+ Misses       2168     2167       -1     

Impacted Files	Coverage Δ
...wvlet/airframe/http/openapi/OpenAPIGenerator.scala	88.93% <100.00%> (+0.09%)	⬆️
...frame-rx/src/main/scala/wvlet/airframe/rx/Rx.scala	85.18% <0.00%> (+1.23%)	⬆️

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