fix(tsdb): Fix variables masked by a declaration (#18129)

Before this commit, the to and from variables were being re-declared in
a block in such a way that the values were not being used.

This patch uses regular assignment so that the values are visable
outside of the block where they're set.

Closes: 18128

tsdb/store_test.go

@@ -1054,7 +1054,7 @@ func testStoreCardinalityDuplicates(t *testing.T, store *Store) {
 			// For other shards we write a random sub-section of all the points.
 			// which will duplicate the series and shouldn't increase the
 			// cardinality.
-			from, to := rand.Intn(len(points)), rand.Intn(len(points))
+			from, to = rand.Intn(len(points)), rand.Intn(len(points))
 			if from > to {
 				from, to = to, from
 			}