[SPARK-23207][SQL][FOLLOW-UP] Use SQLConf.get.enableRadixSort instead of SparkEnv.get.conf.get(SQLConf.RADIX_SORT_ENABLED).

## What changes were proposed in this pull request?

This is a follow-up of apache#20393.
We should read the conf `"spark.sql.sort.enableRadixSort"` from `SQLConf` instead of `SparkConf`, i.e., use `SQLConf.get.enableRadixSort` instead of `SparkEnv.get.conf.get(SQLConf.RADIX_SORT_ENABLED)`, otherwise the config is never read.

## How was this patch tested?

Existing tests.

Closes apache#23046 from ueshin/issues/SPARK-23207/conf.

Authored-by: Takuya UESHIN <ueshin@databricks.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
(cherry picked from commit dad2d82)
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

Author: ueshin

sql/core/src/main/scala/org/apache/spark/sql/execution/exchange/ShuffleExchangeExec.scala

@@ -269,7 +269,7 @@ object ShuffleExchangeExec {
           }
           // The comparator for comparing row hashcode, which should always be Integer.
           val prefixComparator = PrefixComparators.LONG
-          val canUseRadixSort = SparkEnv.get.conf.get(SQLConf.RADIX_SORT_ENABLED)
+          val canUseRadixSort = SQLConf.get.enableRadixSort
           // The prefix computer generates row hashcode as the prefix, so we may decrease the
           // probability that the prefixes are equal when input rows choose column values from a
           // limited range.