ci/codecov: ignore go/extra code coverage

[ptrus]

Since extra is for packages that are not directly part of oasis-core it makes sense skipping those for coverage checks.

Noticed when the coverage increased after removing extra/stats: #3270

[codecov]

Codecov Report

Merging #3272 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3272      +/-   ##
==========================================
+ Coverage   67.73%   67.77%   +0.03%     
==========================================
  Files         371      371              
  Lines       36884    36884              
==========================================
+ Hits        24984    24998      +14     
+ Misses       8712     8707       -5     
+ Partials     3188     3179       -9     

Impacted Files	Coverage Δ
go/storage/mkvs/checkpoint/restorer.go	82.45% <0.00%> (-10.53%)	⬇️
go/oasis-node/cmd/common/metrics/resource.go	84.00% <0.00%> (-8.00%)	⬇️
go/runtime/client/watcher.go	64.61% <0.00%> (-6.16%)	⬇️
...n/crypto/signature/signers/memory/memory_signer.go	71.42% <0.00%> (-4.77%)	⬇️
go/consensus/tendermint/full/services.go	78.90% <0.00%> (-3.91%)	⬇️
go/storage/mkvs/syncer/proof.go	83.92% <0.00%> (-3.58%)	⬇️
go/storage/client/client.go	70.79% <0.00%> (-3.10%)	⬇️
go/worker/storage/committee/checkpoint_sync.go	78.13% <0.00%> (-2.51%)	⬇️
go/storage/mkvs/iterator.go	83.12% <0.00%> (-1.25%)	⬇️
go/staking/api/grpc.go	54.26% <0.00%> (-0.78%)	⬇️
... and 18 more

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