Fix a data race in the loadWAL function caused by reusing the same error var in multiple goroutines (#9259)

Signed-off-by: Callum Styan <callumstyan@gmail.com>

tsdb/head_wal.go

@@ -113,6 +113,7 @@ func (h *Head) loadWAL(r *wal.Reader, multiRef map[uint64]uint64, mmappedChunks
 	wg.Add(1)
 	exemplarsInput = make(chan record.RefExemplar, 300)
 	go func(input <-chan record.RefExemplar) {
+		var err error
 		defer wg.Done()
 		for e := range input {
 			ms := h.series.getByID(e.Ref)