fix: Fix for how the loop sync is done

[paul1r]

What this PR does / why we need it:
Before fix:

go test -race -count=10 . -timeout 30s
==================
WARNING: DATA RACE
Write at 0x00c000213030 by goroutine 493:
  runtime.racewrite()
      <autogenerated>:1 +0x10
  github.com/grafana/loki/v3/pkg/storage/stores/shipper/indexshipper/downloads.(*tableManager).Stop()
      /Users/progers/dev/src/github.com/grafana/loki/pkg/storage/stores/shipper/indexshipper/downloads/table_manager.go:156 +0x50
  github.com/grafana/loki/v3/pkg/storage/stores/shipper/indexshipper/downloads.buildTestTableManager.func2()
      /Users/progers/dev/src/github.com/grafana/loki/pkg/storage/stores/shipper/indexshipper/downloads/table_manager_test.go:68 +0x3c
  runtime.deferreturn()
      /opt/homebrew/Cellar/go/1.22.2/libexec/src/runtime/panic.go:602 +0x5c
  testing.tRunner()
      /opt/homebrew/Cellar/go/1.22.2/libexec/src/testing/testing.go:1689 +0x180
  testing.(*T).Run.gowrap1()
      /opt/homebrew/Cellar/go/1.22.2/libexec/src/testing/testing.go:1742 +0x40

Previous read at 0x00c000213030 by goroutine 494:
  runtime.raceread()
      <autogenerated>:1 +0x10
  github.com/grafana/loki/v3/pkg/storage/stores/shipper/indexshipper/downloads.(*tableManager).loop()
      /Users/progers/dev/src/github.com/grafana/loki/pkg/storage/stores/shipper/indexshipper/downloads/table_manager.go:120 +0x50
  github.com/grafana/loki/v3/pkg/storage/stores/shipper/indexshipper/downloads.NewTableManager.gowrap1()
      /Users/progers/dev/src/github.com/grafana/loki/pkg/storage/stores/shipper/indexshipper/downloads/table_manager.go:114 +0x34

Goroutine 493 (running) created at:
  testing.(*T).Run()
      /opt/homebrew/Cellar/go/1.22.2/libexec/src/testing/testing.go:1742 +0x5e4
  testing.runTests.func1()
      /opt/homebrew/Cellar/go/1.22.2/libexec/src/testing/testing.go:2161 +0x80
  testing.tRunner()
      /opt/homebrew/Cellar/go/1.22.2/libexec/src/testing/testing.go:1689 +0x180
  testing.runTests()
      /opt/homebrew/Cellar/go/1.22.2/libexec/src/testing/testing.go:2159 +0x6e0
  testing.(*M).Run()
      /opt/homebrew/Cellar/go/1.22.2/libexec/src/testing/testing.go:2027 +0xb74
  main.main()
      _testmain.go:69 +0x294

Goroutine 494 (finished) created at:
  github.com/grafana/loki/v3/pkg/storage/stores/shipper/indexshipper/downloads.NewTableManager()
      /Users/progers/dev/src/github.com/grafana/loki/pkg/storage/stores/shipper/indexshipper/downloads/table_manager.go:114 +0x400
  github.com/grafana/loki/v3/pkg/storage/stores/shipper/indexshipper/downloads.buildTestTableManager()
      /Users/progers/dev/src/github.com/grafana/loki/pkg/storage/stores/shipper/indexshipper/downloads/table_manager_test.go:62 +0x2fc
  github.com/grafana/loki/v3/pkg/storage/stores/shipper/indexshipper/downloads.TestTableManager_cleanupCache()
      /Users/progers/dev/src/github.com/grafana/loki/pkg/storage/stores/shipper/indexshipper/downloads/table_manager_test.go:103 +0x44
  testing.tRunner()
      /opt/homebrew/Cellar/go/1.22.2/libexec/src/testing/testing.go:1689 +0x180
  testing.(*T).Run.gowrap1()
      /opt/homebrew/Cellar/go/1.22.2/libexec/src/testing/testing.go:1742 +0x40
==================
--- FAIL: TestTableManager_cleanupCache (0.00s)
    testing.go:1398: race detected during execution of test
FAIL
FAIL	github.com/grafana/loki/v3/pkg/storage/stores/shipper/indexshipper/downloads	21.587s
FAIL

After fix:

 go test -race -count=10 . -timeout 30s
ok  	github.com/grafana/loki/v3/pkg/storage/stores/shipper/indexshipper/downloads	22.217s
progers@Pauls-MacBook-Pro downloads % 

Which issue(s) this PR fixes:
Relates to #8586

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
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