[SPARK-26144][BUILD] build/mvn should detect scala.version based on scala.binary.version

## What changes were proposed in this pull request?

Currently, `build/mvn` downloads and uses **Scala 2.12.7** in `Scala-2.11` Jenkins job. The root cause is `build/mvn` got the first match from `pom.xml` blindly.

- https://amplab.cs.berkeley.edu/jenkins/view/Spark%20QA%20Test%20(Dashboard)/job/spark-master-test-maven-hadoop-2.7-ubuntu-scala-2.11/6/consoleFull
```
exec: curl -s -L https://downloads.lightbend.com/zinc/0.3.15/zinc-0.3.15.tgz
exec: curl -s -L https://downloads.lightbend.com/scala/2.12.7/scala-2.12.7.tgz
exec: curl -s -L https://www.apache.org/dyn/closer.lua?action=download&filename=/maven/maven-3/3.5.4/binaries/apache-maven-3.5.4-bin.tar.gz
```

## How was this patch tested?

Manual.
```
$ build/mvn clean
exec: curl --progress-bar -L https://downloads.lightbend.com/scala/2.12.7/scala-2.12.7.tgz
...
$ git clean -fdx
$ dev/change-scala-version.sh 2.11
$ build/mvn clean
exec: curl --progress-bar -L https://downloads.lightbend.com/scala/2.11.12/scala-2.11.12.tgz
```

Closes apache#23118 from dongjoon-hyun/SPARK-26144.

Authored-by: Dongjoon Hyun <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: dongjoon-hyun

build/mvn

@@ -116,7 +116,8 @@ install_zinc() {
 # the build/ folder
 install_scala() {
   # determine the Scala version used in Spark
-  local scala_version=`grep "scala.version" "${_DIR}/../pom.xml" | head -n1 | awk -F '[<>]' '{print $3}'`
+  local scala_binary_version=`grep "scala.binary.version" "${_DIR}/../pom.xml" | head -n1 | awk -F '[<>]' '{print $3}'`
+  local scala_version=`grep "scala.version" "${_DIR}/../pom.xml" | grep ${scala_binary_version} | head -n1 | awk -F '[<>]' '{print $3}'`
   local scala_bin="${_DIR}/scala-${scala_version}/bin/scala"
   local TYPESAFE_MIRROR=${TYPESAFE_MIRROR:-https://downloads.lightbend.com}