go/worker/common/committee: group version should be the epoch block

[ptrus]

Discovered while testing: #3322

Note: in the other place where we compute current version (runtime client) we already handle this case correctly.

[codecov]

Codecov Report

Merging #3347 into master will decrease coverage by 0.39%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master    #3347      +/-   ##
==========================================
- Coverage   65.44%   65.04%   -0.40%     
==========================================
  Files         371      371              
  Lines       33151    33154       +3     
==========================================
- Hits        21695    21566     -129     
- Misses       8261     8375     +114     
- Partials     3195     3213      +18     

Impacted Files	Coverage Δ
go/worker/common/committee/group.go	77.06% <60.00%> (-0.62%)	⬇️
go/storage/mkvs/checkpoint/checkpointer.go	64.75% <0.00%> (-15.58%)	⬇️
go/worker/storage/service_external.go	40.74% <0.00%> (-11.12%)	⬇️
go/runtime/host/sandbox/sandbox.go	64.96% <0.00%> (-10.63%)	⬇️
go/consensus/tendermint/api/errors.go	90.00% <0.00%> (-10.00%)	⬇️
go/consensus/tendermint/abci/state/state.go	54.54% <0.00%> (-9.10%)	⬇️
go/common/grpc/policy/policy.go	62.12% <0.00%> (-7.58%)	⬇️
go/staking/api/grpc.go	51.26% <0.00%> (-6.73%)	⬇️
go/consensus/tendermint/abci/mux.go	49.90% <0.00%> (-5.07%)	⬇️
go/consensus/tendermint/apps/staking/query.go	35.71% <0.00%> (-4.77%)	⬇️
... and 22 more

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