Ignore .terragrunt-cache dirs.

[lkysow]

Fixes #487

[codecov]

Codecov Report

Merging #517 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #517      +/-   ##
==========================================
+ Coverage   72.17%   72.18%   +0.01%     
==========================================
  Files          62       62              
  Lines        4507     4509       +2     
==========================================
+ Hits         3253     3255       +2     
  Misses       1011     1011              
  Partials      243      243

Impacted Files	Coverage Δ
server/events/pending_plan_finder.go	81.57% <100%> (+1.02%)	⬆️

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[sryabkov]

@lkysow 👍 , thank you, but please see a more generic solution proposal described in #487