[Gradle] KotlinToolRunner: Capture System.getProperties() as List

This is done to avoid a potential `ConcurrentModificationException`:

```
Caused by: java.util.ConcurrentModificationException
  at java.util.Hashtable$Enumerator.next(Hashtable.java:1387)
  at org.gradle.internal.classpath.Instrumented$DecoratingEntrySet$1.computeNext(Instrumented.java:349)
  at org.gradle.internal.classpath.Instrumented$DecoratingEntrySet$1.computeNext(Instrumented.java:343)
  at com.google.common.collect.AbstractIterator.tryToComputeNext(AbstractIterator.java:141)
  at com.google.common.collect.AbstractIterator.hasNext(AbstractIterator.java:136)
  at kotlin.sequences.TransformingSequence$iterator$1.hasNext(Sequences.kt:176)
  at kotlin.sequences.FilteringSequence$iterator$1.calcNext(Sequences.kt:131)
  at kotlin.sequences.FilteringSequence$iterator$1.hasNext(Sequences.kt:156)
  at kotlin.collections.MapsKt__MapsKt.putAll(Maps.kt:419)
  at kotlin.collections.MapsKt__MapsKt.toMap(Maps.kt:587)
  at kotlin.collections.MapsKt__MapsKt.toMap(Maps.kt:581)
  at org.jetbrains.kotlin.compilerRunner.KotlinToolRunner.runViaExec(KotlinToolRunner.kt:95)
  at org.jetbrains.kotlin.compilerRunner.KotlinToolRunner.run(KotlinToolRunner.kt:85)
```

libraries/tools/kotlin-gradle-plugin/src/common/kotlin/org/jetbrains/kotlin/compilerRunner/KotlinToolRunner.kt

@@ -12,6 +12,7 @@ import org.jetbrains.kotlin.konan.target.HostManager
 import java.io.File
 import java.lang.reflect.InvocationTargetException
 import java.net.URLClassLoader
+import java.util.*
 import java.util.concurrent.ConcurrentHashMap
 
 // Note: this class is public because it is used in the K/N build infrastructure.
@@ -88,7 +89,10 @@ abstract class KotlinToolRunner(
     private fun runViaExec(args: List<String>) {
         val transformedArgs = transformArgs(args)
         val classpath = project.files(classpath)
-        val systemProperties = System.getProperties().asSequence()
+        val systemProperties = System.getProperties()
+            /* Capture 'System.getProperties()' as List to avoid potential 'ConcurrentModificationException' */
+            .toListSynchronized()
+            .asSequence()
             .map { (k, v) -> k.toString() to v.toString() }
             .filter { (k, _) -> k !in execSystemPropertiesBlacklist }
             .escapeQuotesForWindows()
@@ -173,5 +177,10 @@ abstract class KotlinToolRunner(
                 any { it == '"' || it.isWhitespace() } -> '"' + escapeStringCharacters(this) + '"'
                 else -> this
             }
+
+        /**
+         * Safely enter the [Properties] monitor and capture the current values
+         */
+        private fun Properties.toListSynchronized(): List<Pair<Any, Any>> = synchronized(this) { toList() }
     }
 }