go/txsource: fix queries workload earliest committee epoch

[ptrus]

Keymanager actually registers in epoch 1, not sure if this is due to a recent change (e.g. the WaitFor() changes) or it was always like that

This was a reason for a failure in one of the recent longtests runs

[codecov]

Codecov Report

Merging #3001 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3001      +/-   ##
==========================================
- Coverage   68.46%   68.45%   -0.02%     
==========================================
  Files         360      360              
  Lines       35382    35382              
==========================================
- Hits        24226    24222       -4     
- Misses       8011     8028      +17     
+ Partials     3145     3132      -13     

Impacted Files	Coverage Δ
go/storage/mkvs/writelog/iterator.go	76.36% <0.00%> (-16.37%)	⬇️
go/worker/compute/executor/committee/state.go	74.07% <0.00%> (-14.82%)	⬇️
go/consensus/tendermint/api/api.go	74.54% <0.00%> (-14.55%)	⬇️
go/worker/storage/service_external.go	40.86% <0.00%> (-6.46%)	⬇️
go/worker/keymanager/handler.go	67.92% <0.00%> (-3.78%)	⬇️
go/storage/api/grpc.go	66.88% <0.00%> (-3.68%)	⬇️
go/storage/mkvs/prefetch.go	59.32% <0.00%> (-3.39%)	⬇️
go/worker/storage/committee/node.go	69.97% <0.00%> (-3.17%)	⬇️
go/worker/compute/executor/committee/node.go	61.03% <0.00%> (-1.92%)	⬇️
go/storage/mkvs/checkpoint/checkpointer.go	63.70% <0.00%> (-1.49%)	⬇️
... and 24 more

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