Race in syncer

Race in syncer

```
WARNING: DATA RACE
Write at 0x00c00014e168 by goroutine 2559:
github.com/celestiaorg/go-header/sync.(*Syncer[go.shape.*uint8]).Start()
/Users/rene/go/src/github.com/renaynay/go-header/sync/syncer.go:101 +0x60
github.com/celestiaorg/go-header/sync.TestSyncer_TailHashOverHeight()
/Users/rene/go/src/github.com/renaynay/go-header/sync/syncer_tail_test.go:59 +0x630
testing.tRunner()
/opt/homebrew/Cellar/go/1.24.3/libexec/src/testing/testing.go:1792 +0x180
testing.(*T).Run.gowrap1()
/opt/homebrew/Cellar/go/1.24.3/libexec/src/testing/testing.go:1851 +0x40

Previous read at 0x00c00014e168 by goroutine 2561:
github.com/celestiaorg/go-header/sync.(*Syncer[go.shape.*uint8]).syncLoop()
/Users/rene/go/src/github.com/renaynay/go-header/sync/syncer.go:228 +0xf0
github.com/celestiaorg/go-header/sync.(*Syncer[go.shape.*uint8]).Start.gowrap1()
/Users/rene/go/src/github.com/renaynay/go-header/sync/syncer.go:143 +0x40
```