common/txmgr: initialize map before goroutines race ahead

[jmank88]

This PR reorders the statements in *Broadcaster.startInternal() so that the nextSequenceMap is initialized before the spawned goroutines have an opportunity to race ahead and access it.

$ go test ./core/chains/evm/txmgr/ -race -count 5
panic: assignment to entry in nil map

goroutine 1805 [running]:
github.com/smartcontractkit/chainlink/v2/common/txmgr.(*Broadcaster[...]).GetNextSequence(0x43dfdc0, {0x43b5538, 0xc002798410}, {0x80, 0xf7, 0x22, 0x1b, 0x6a, 0x3a, 0xce, ...})
        /home/jordan/chainlink/common/txmgr/broadcaster.go:826 +0x5ed
github.com/smartcontractkit/chainlink/v2/common/txmgr.(*Broadcaster[...]).SyncSequence(0x43dfdc0, {0x43b5538, 0xc002798410}, {0x80, 0xf7, 0x22, 0x1b, 0x6a, 0x3a, 0xce, ...})
        /home/jordan/chainlink/common/txmgr/broadcaster.go:403 +0x149
github.com/smartcontractkit/chainlink/v2/common/txmgr.(*Broadcaster[...]).monitorTxs(0x43dfdc0, {0x80, 0xf7, 0x22, 0x1b, 0x6a, 0x3a, 0xce, 0x50, 0xce, ...}, ...)
        /home/jordan/chainlink/common/txmgr/broadcaster.go:347 +0x3ee
created by github.com/smartcontractkit/chainlink/v2/common/txmgr.(*Broadcaster[...]).startInternal in goroutine 1804
        /home/jordan/chainlink/common/txmgr/broadcaster.go:228 +0x7cf
FAIL    github.com/smartcontractkit/chainlink/v2/core/chains/evm/txmgr  117.452s
FAIL

[github-actions]

I see that you haven't updated any CHANGELOG files. Would it make sense to do so?

[cl-sonarqube-production]

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[amit-momin]

Thanks for handling this!