internal/era: update block index format to be based on record offset (#28959)

As mentioned in #26621, the block index format for era1 is not in line with the regular era block index. This change modifies the index so all relative offsets are based against the beginning of the block index record.

cmd/utils/history_test.go

@@ -134,7 +134,7 @@ func TestHistoryImportAndExport(t *testing.T) {
 			for j := 0; it.Next(); j++ {
 				n := i*int(step) + j
 				if it.Error() != nil {
-					t.Fatalf("error reading block entry %d: %v", n, err)
+					t.Fatalf("error reading block entry %d: %v", n, it.Error())
 				}
 				block, receipts, err := it.BlockAndReceipts()
 				if err != nil {