.buildkite/pipeline: require large agents for coverage step

[ptrus]

temoporary, until: #2644

[codecov]

Codecov Report

Merging #2645 into master will decrease coverage by 0.24%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2645      +/-   ##
==========================================
- Coverage   62.99%   62.74%   -0.25%     
==========================================
  Files         364      364              
  Lines       34330    34330              
==========================================
- Hits        21626    21541      -85     
- Misses      10061    10115      +54     
- Partials     2643     2674      +31

Impacted Files	Coverage Δ
go/storage/mkvs/urkel/writelog/iterator.go	76.36% <0%> (-16.37%)	⬇️
go/worker/storage/service_external.go	40.44% <0%> (-11.24%)	⬇️
go/worker/common/p2p/p2p.go	61.71% <0%> (-10.37%)	⬇️
go/control/debug.go	67.74% <0%> (-6.46%)	⬇️
go/common/cbor/codec.go	64.7% <0%> (-5.89%)	⬇️
go/common/grpc/auth/auth.go	88.88% <0%> (-5.56%)	⬇️
go/storage/metrics.go	70.53% <0%> (-5.36%)	⬇️
go/storage/api/grpc.go	66.17% <0%> (-4.84%)	⬇️
go/worker/storage/committee/node.go	76.86% <0%> (-4.26%)	⬇️
go/oasis-node/cmd/debug/storage/storage.go	45.08% <0%> (-3.47%)	⬇️
... and 8 more

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