backport/ci: Bump grcov version to 0.8.7

[thanethomson]

Backport of #1098

[codecov-commenter]

Codecov Report

Merging #1099 (5c86dcc) into v0.23.x (2354801) will increase coverage by 0.0%.
The diff coverage is n/a.

❗ Current head 5c86dcc differs from pull request most recent head cb09872. Consider uploading reports for the commit cb09872 to get more accurate results

@@           Coverage Diff           @@
##           v0.23.x   #1099   +/-   ##
=======================================
  Coverage     67.3%   67.4%           
=======================================
  Files          212     212           
  Lines        20201   20201           
=======================================
+ Hits         13614   13617    +3     
+ Misses        6587    6584    -3     

Impacted Files	Coverage Δ
light-client-verifier/src/types.rs	38.5% <0.0%> (+0.2%)	⬆️
testgen/src/header.rs	85.0% <0.0%> (+0.5%)	⬆️
testgen/src/vote.rs	84.8% <0.0%> (+0.8%)	⬆️

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