fix: Add data race protection for the tm.wg var

[paul1r]

What this PR does / why we need it:
Moves access to the table_manager.wg var behind the existing tablesMtx mutex
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
• CHANGELOG.md updated
  • If the change is worth mentioning in the release notes, add add-to-release-notes label
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• For Helm chart changes bump the Helm chart version in production/helm/loki/Chart.yaml and update production/helm/loki/CHANGELOG.md and production/helm/loki/README.md. Example PR
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

[paul1r]

Previously:

==================
WARNING: DATA RACE
Write at 0x00c0005cb2a0 by goroutine 163:
  runtime.racewrite()
      <autogenerated>:1 +0x10
  github.com/grafana/loki/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:154 +0x60
  github.com/grafana/loki/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/1.21.6/libexec/src/runtime/panic.go:477 +0x34
  testing.tRunner()
      /opt/homebrew/Cellar/go/1.21.6/libexec/src/testing/testing.go:1595 +0x1b0
  testing.(*T).Run.func1()
      /opt/homebrew/Cellar/go/1.21.6/libexec/src/testing/testing.go:1648 +0x40

Previous read at 0x00c0005cb2a0 by goroutine 164:
  runtime.raceread()
      <autogenerated>:1 +0x10
  github.com/grafana/loki/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:119 +0x40
  github.com/grafana/loki/pkg/storage/stores/shipper/indexshipper/downloads.NewTableManager.func1()
      /Users/progers/dev/src/github.com/grafana/loki/pkg/storage/stores/shipper/indexshipper/downloads/table_manager.go:114 +0x34

Goroutine 163 (running) created at:
  testing.(*T).Run()
      /opt/homebrew/Cellar/go/1.21.6/libexec/src/testing/testing.go:1648 +0x5e8
  testing.runTests.func1()
      /opt/homebrew/Cellar/go/1.21.6/libexec/src/testing/testing.go:2054 +0x80
  testing.tRunner()
      /opt/homebrew/Cellar/go/1.21.6/libexec/src/testing/testing.go:1595 +0x1b0
  testing.runTests()
      /opt/homebrew/Cellar/go/1.21.6/libexec/src/testing/testing.go:2052 +0x6e4
  testing.(*M).Run()
      /opt/homebrew/Cellar/go/1.21.6/libexec/src/testing/testing.go:1925 +0x9ec
  main.main()
      _testmain.go:69 +0x294

Goroutine 164 (finished) created at:
  github.com/grafana/loki/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 +0x404
  github.com/grafana/loki/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/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/1.21.6/libexec/src/testing/testing.go:1595 +0x1b0
  testing.(*T).Run.func1()
      /opt/homebrew/Cellar/go/1.21.6/libexec/src/testing/testing.go:1648 +0x40
==================
    testing.go:1465: race detected during execution of test
--- FAIL: TestTableManager_cleanupCache (0.00s)

[paul1r]

After fix:

go test -race -count=1000  . -run TestTableManager_cleanupCache
ok  	github.com/grafana/loki/pkg/storage/stores/shipper/indexshipper/downloads	1.916s