Only post a code coverage patch status to PRs

[schalkms]

This affects the codecov coverage reports.
Patch coverage reports should only run on PRs.
It provides information concerning how well the changes made in a PR were tested.
Previously, many patch reports have failed already merged PRs on the master branch.
The total coverage is not affected by this commit.
See:
https://docs.codecov.io/docs/commit-status#section-only-pulls

[codecov-io]

Codecov Report

Merging #2402 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #2402      +/-   ##
============================================
+ Coverage     82.87%   82.88%   +0.01%     
+ Complexity     2148     2147       -1     
============================================
  Files           353      353              
  Lines          6119     6123       +4     
  Branches       1118     1116       -2     
============================================
+ Hits           5071     5075       +4     
  Misses          475      475              
  Partials        573      573

Impacted Files	Coverage Δ	Complexity Δ
.../arturbosch/detekt/rules/style/UnnecessaryApply.kt	79.31% <0%> (-1.94%)	6% <0%> (ø)
...les/style/ExplicitCollectionElementAccessMethod.kt	90.32% <0%> (-0.31%)	16% <0%> (-1%)
...n/io/gitlab/arturbosch/detekt/rules/LinesOfCode.kt	100% <0%> (ø)	0% <0%> (ø)	⬇️
...ain/kotlin/io/gitlab/arturbosch/detekt/api/Debt.kt	100% <0%> (ø)	6% <0%> (ø)	⬇️
...b/arturbosch/detekt/cli/FilteredDetectionResult.kt	100% <0%> (ø)	2% <0%> (ø)	⬇️
...itlab/arturbosch/detekt/core/ProcessingSettings.kt	90.62% <0%> (+0.14%)	18% <0%> (ø)	⬇️
...n/io/gitlab/arturbosch/detekt/cli/out/XmlEscape.kt	64.28% <0%> (+0.25%)	3% <0%> (ø)	⬇️
...rturbosch/detekt/formatting/OffsetsToLineColumn.kt	82.14% <0%> (+0.66%)	0% <0%> (ø)	⬇️
...in/kotlin/io/gitlab/arturbosch/detekt/api/Issue.kt	87.5% <0%> (+1.78%)	5% <0%> (ø)	⬇️
...b/arturbosch/detekt/api/internal/DetektPomModel.kt	68.75% <0%> (+2.08%)	5% <0%> (ø)	⬇️
... and 1 more

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