Move gitlint from Buildkite to GitHub Actions

[tjanez]

And related smaller improvements.

[codecov]

Codecov Report

Merging #2692 into master will decrease coverage by 0.2%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2692      +/-   ##
==========================================
- Coverage    63.3%   63.09%   -0.21%     
==========================================
  Files         377      377              
  Lines       35448    35448              
==========================================
- Hits        22439    22367      -72     
- Misses      10214    10273      +59     
- Partials     2795     2808      +13

Impacted Files	Coverage Δ
go/worker/common/host/interface.go	38.46% <0%> (-15.39%)	⬇️
go/worker/compute/executor/committee/state.go	74.07% <0%> (-11.12%)	⬇️
go/worker/common/host/protocol/protocol.go	65.67% <0%> (-6.72%)	⬇️
go/consensus/api/grpc.go	57.02% <0%> (-6.62%)	⬇️
go/common/grpc/proxy/proxy.go	58.87% <0%> (-5.61%)	⬇️
go/common/grpc/auth/auth.go	88.88% <0%> (-5.56%)	⬇️
go/worker/compute/executor/committee/node.go	60.82% <0%> (-5.32%)	⬇️
go/storage/api/root_cache.go	72.41% <0%> (-4.6%)	⬇️
go/worker/storage/service_external.go	47.31% <0%> (-4.31%)	⬇️
go/runtime/history/prune.go	82.14% <0%> (-3.58%)	⬇️
... and 20 more

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