go: Disable additional signature verification for migration

[Yawning]

For the purpose of migration.

Predictably, disabling this sort of thing is error prone. :/

[codecov]

Codecov Report

Merging #2652 into master will decrease coverage by 0.02%.
The diff coverage is 31.57%.

@@            Coverage Diff             @@
##           master    #2652      +/-   ##
==========================================
- Coverage   62.92%   62.89%   -0.03%     
==========================================
  Files         365      365              
  Lines       34458    34468      +10     
==========================================
- Hits        21682    21679       -3     
- Misses      10077    10085       +8     
- Partials     2699     2704       +5

Impacted Files	Coverage Δ
go/oasis-node/cmd/debug/fixgenesis/fixgenesis.go	8.53% <0%> (ø)	⬆️
go/consensus/tendermint/seed.go	70.65% <16.66%> (-2.08%)	⬇️
go/consensus/tendermint/tendermint.go	66.07% <37.5%> (-0.97%)	⬇️
go/registry/api/api.go	36.82% <50%> (ø)	⬆️
go/consensus/api/grpc.go	57.02% <0%> (-6.62%)	⬇️
go/worker/storage/service_external.go	47.19% <0%> (-4.5%)	⬇️
go/worker/common/p2p/p2p.go	64.41% <0%> (-3.16%)	⬇️
go/worker/common/committee/group.go	79.33% <0%> (-0.67%)	⬇️
go/roothash/tests/tester.go	95.54% <0%> (-0.64%)	⬇️
... and 6 more

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