loadgenerator: correctly handle AccountMnemonicList

[cce]

Summary

Fix bug in loadgenerator when handling AccountMnemonicList.

[algolucky]

confirm this is working as expected https://betanet.algoexplorer.io/block/20118672

usually ~90 txns per round on betanet

[algolucky]

another block to show it's working: https://betanet.algoexplorer.io/block/20119162

[codecov]

Codecov Report

Merging #4454 (930bd07) into master (924d2fb) will increase coverage by 3.68%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4454      +/-   ##
==========================================
+ Coverage   51.51%   55.19%   +3.68%     
==========================================
  Files         398      398              
  Lines       50165    50165              
==========================================
+ Hits        25841    27688    +1847     
+ Misses      21934    20164    -1770     
+ Partials     2390     2313      -77     

Impacted Files	Coverage Δ
cmd/algoh/blockWatcher.go	77.77% <0.00%> (-3.18%)	⬇️
network/wsNetwork.go	64.82% <0.00%> (+0.28%)	⬆️
network/requestTracker.go	71.12% <0.00%> (+0.43%)	⬆️
ledger/internal/prefetcher/prefetcher.go	68.18% <0.00%> (+0.45%)	⬆️
node/node.go	4.43% <0.00%> (+0.47%)	⬆️
daemon/algod/api/server/v2/dryrun.go	70.82% <0.00%> (+0.60%)	⬆️
agreement/pseudonode.go	70.07% <0.00%> (+0.72%)	⬆️
ledger/catchpointwriter.go	57.65% <0.00%> (+0.90%)	⬆️
crypto/merkletrie/cache.go	89.02% <0.00%> (+1.18%)	⬆️
ledger/blockdb.go	56.25% <0.00%> (+1.25%)	⬆️
... and 110 more

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