make workers field atomic

[AskAlexSharov]

==================
Write at 0x00c0495a9a58 by goroutine 18408854:
  github.com/erigontech/erigon/turbo/snapshotsync/freezeblocks.(*BlockRetire).SetWorkers()
      /home/ubuntu/erigon/turbo/snapshotsync/freezeblocks/block_snapshots.go:201 +0x37
  github.com/erigontech/erigon/eth/stagedsync.SnapshotsPrune()
      /home/ubuntu/erigon/eth/stagedsync/stage_snapshots.go:589 +0x3a7
  github.com/erigontech/erigon/eth/stagedsync.PipelineStages.func3()
      /home/ubuntu/erigon/eth/stagedsync/default_stages.go:205 +0x13b
  github.com/erigontech/erigon/eth/stagedsync.(*Sync).pruneStage()
      /home/ubuntu/erigon/eth/stagedsync/sync.go:593 +0x327
  github.com/erigontech/erigon/eth/stagedsync.(*Sync).RunPrune()
      /home/ubuntu/erigon/eth/stagedsync/sync.go:462 +0x2ac
  github.com/erigontech/erigon/turbo/execution/eth1.(*EthereumExecutionModule).runPostForkchoiceInBackground.func1.1()
      /home/ubuntu/erigon/turbo/execution/eth1/forkchoice.go:569 +0xb8
  github.com/erigontech/erigon-lib/kv/temporal.(*DB).Update()
      /home/ubuntu/erigon/erigon-lib/kv/temporal/kv_temporal.go:124 +0x18e
  github.com/erigontech/erigon/turbo/execution/eth1.(*EthereumExecutionModule).runPostForkchoiceInBackground.func1()
      /home/ubuntu/erigon/turbo/execution/eth1/forkchoice.go:568 +0x325

Previous read at 0x00c0495a9a58 by goroutine 18408728:
  github.com/erigontech/erigon/turbo/snapshotsync/freezeblocks.(*BlockRetire).retireBlocks()
      /home/ubuntu/erigon/turbo/snapshotsync/freezeblocks/block_snapshots.go:269 +0x1be
  github.com/erigontech/erigon/turbo/snapshotsync/freezeblocks.(*BlockRetire).RetireBlocks()
      /home/ubuntu/erigon/turbo/snapshotsync/freezeblocks/block_snapshots.go:439 +0x24a
  github.com/erigontech/erigon/turbo/snapshotsync/freezeblocks.(*BlockRetire).RetireBlocksInBackground.func1()
      /home/ubuntu/erigon/turbo/snapshotsync/freezeblocks/block_snapshots.go:398 +0x377

Goroutine 18408854 (running) created at:
  github.com/erigontech/erigon/turbo/execution/eth1.(*EthereumExecutionModule).runPostForkchoiceInBackground()
      /home/ubuntu/erigon/turbo/execution/eth1/forkchoice.go:560 +0xd6
  github.com/erigontech/erigon/turbo/execution/eth1.(*EthereumExecutionModule).updateForkChoice()
      /home/ubuntu/erigon/turbo/execution/eth1/forkchoice.go:546 +0x492a
  github.com/erigontech/erigon/turbo/execution/eth1.(*EthereumExecutionModule).UpdateForkChoice.gowrap1()
      /home/ubuntu/erigon/turbo/execution/eth1/forkchoice.go:130 +0x133

Goroutine 18408728 (running) created at:
  github.com/erigontech/erigon/turbo/snapshotsync/freezeblocks.(*BlockRetire).RetireBlocksInBackground()
      /home/ubuntu/erigon/turbo/snapshotsync/freezeblocks/block_snapshots.go:386 +0x250
  github.com/erigontech/erigon/eth/stagedsync.SnapshotsPrune()
      /home/ubuntu/erigon/eth/stagedsync/stage_snapshots.go:592 +0x6f8
  github.com/erigontech/erigon/eth/stagedsync.PipelineStages.func3()
      /home/ubuntu/erigon/eth/stagedsync/default_stages.go:205 +0x13b
  github.com/erigontech/erigon/eth/stagedsync.(*Sync).pruneStage()
      /home/ubuntu/erigon/eth/stagedsync/sync.go:593 +0x327
  github.com/erigontech/erigon/eth/stagedsync.(*Sync).RunPrune()
      /home/ubuntu/erigon/eth/stagedsync/sync.go:462 +0x2ac
  github.com/erigontech/erigon/turbo/execution/eth1.(*EthereumExecutionModule).runPostForkchoiceInBackground.func1.1()
      /home/ubuntu/erigon/turbo/execution/eth1/forkchoice.go:569 +0xb8
  github.com/erigontech/erigon-lib/kv/temporal.(*DB).Update()
      /home/ubuntu/erigon/erigon-lib/kv/temporal/kv_temporal.go:124 +0x18e
  github.com/erigontech/erigon/turbo/execution/eth1.(*EthereumExecutionModule).runPostForkchoiceInBackground.func1()
      /home/ubuntu/erigon/turbo/execution/eth1/forkchoice.go:568 +0x325
==================