go/oasis-test-runner/txsource: Generate a new random seed on each run

[kostko]

I remembered we don't actually change the seed 😁

[codecov]

Codecov Report

Merging #2849 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2849      +/-   ##
==========================================
+ Coverage   67.36%   67.41%   +0.04%     
==========================================
  Files         344      344              
  Lines       33411    33411              
==========================================
+ Hits        22508    22523      +15     
+ Misses       7996     7992       -4     
+ Partials     2907     2896      -11     

Impacted Files	Coverage Δ
go/consensus/tendermint/roothash/roothash.go	62.50% <0.00%> (-5.41%)	⬇️
go/consensus/tendermint/abci/timer.go	67.01% <0.00%> (-5.16%)	⬇️
go/storage/mkvs/remove.go	85.85% <0.00%> (-4.05%)	⬇️
go/storage/client/client.go	75.33% <0.00%> (-1.77%)	⬇️
go/storage/mkvs/cache.go	81.27% <0.00%> (-1.71%)	⬇️
go/storage/mkvs/iterator.go	83.12% <0.00%> (-1.25%)	⬇️
go/common/grpc/grpc.go	79.42% <0.00%> (-1.24%)	⬇️
go/runtime/transaction/transaction.go	74.69% <0.00%> (-1.21%)	⬇️
go/consensus/tendermint/tendermint.go	67.24% <0.00%> (-0.54%)	⬇️
go/worker/registration/worker.go	66.73% <0.00%> (-0.42%)	⬇️
... and 18 more

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