[SPARK-19571][R] Fix SparkR test break on Windows via AppVeyor

## What changes were proposed in this pull request?

It seems wintuils for Hadoop 2.6.5 not exiting for now in https://github.com/steveloughran/winutils

This breaks the tests in SparkR on Windows so this PR proposes to use winutils built by Hadoop 2.6.4 for now.

## How was this patch tested?

Manually via AppVeyor

**Before**

https://ci.appveyor.com/project/spark-test/spark/build/627-r-test-break

**After**

https://ci.appveyor.com/project/spark-test/spark/build/629-r-test-break

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #16927 from HyukjinKwon/spark-r-windows-break.

Author: HyukjinKwon

dev/appveyor-install-dependencies.ps1

@@ -95,7 +95,7 @@ $env:MAVEN_OPTS = "-Xmx2g -XX:MaxPermSize=512M -XX:ReservedCodeCacheSize=512m"
 Pop-Location
 
 # ========================== Hadoop bin package
-$hadoopVer = "2.6.5"
+$hadoopVer = "2.6.4"
 $hadoopPath = "$tools\hadoop"
 if (!(Test-Path $hadoopPath)) {
     New-Item -ItemType Directory -Force -Path $hadoopPath | Out-Null