[SPARK-22852][BUILD] Exclude -Xlint:unchecked from sbt javadoc flags

## What changes were proposed in this pull request?

Moves the -Xlint:unchecked flag in the sbt build configuration from Compile to (Compile, compile) scope, allowing publish and publishLocal commands to work.

## How was this patch tested?

Successfully published the spark-launcher subproject from within sbt successfully, where it fails without this patch.

Author: Erik LaBianca <[email protected]>

Closes #20040 from easel/javadoc-xlint.

Author: easel

project/SparkBuild.scala

@@ -239,14 +239,14 @@ object SparkBuild extends PomBuild {
 
     javacOptions in Compile ++= Seq(
       "-encoding", "UTF-8",
-      "-source", javacJVMVersion.value,
-      "-Xlint:unchecked"
+      "-source", javacJVMVersion.value
     ),
-    // This -target option cannot be set in the Compile configuration scope since `javadoc` doesn't
-    // play nicely with it; see https://github.com/sbt/sbt/issues/355#issuecomment-3817629 for
-    // additional discussion and explanation.
+    // This -target and Xlint:unchecked options cannot be set in the Compile configuration scope since
+    // `javadoc` doesn't play nicely with them; see https://github.com/sbt/sbt/issues/355#issuecomment-3817629
+    // for additional discussion and explanation.
     javacOptions in (Compile, compile) ++= Seq(
-      "-target", javacJVMVersion.value
+      "-target", javacJVMVersion.value,
+      "-Xlint:unchecked"
     ),
 
     scalacOptions in Compile ++= Seq(