Let build cache integration be enabled via system property instead of project property

And prove it can be enabled via `$GRADLE_HOME/gradle.properties`.

See #1032

Author: bamboo

subprojects/provider/src/main/kotlin/org/gradle/kotlin/dsl/cache/BuildServices.kt

@@ -33,7 +33,7 @@ object BuildServices {
     ): ScriptCache {
 
         val hasBuildCacheIntegration =
-            startParameters.isBuildCacheEnabled && startParameters.isKotlinDslBuildCacheEnabled
+            startParameters.isBuildCacheEnabled && isKotlinDslBuildCacheEnabled
 
         return ScriptCache(
             cacheRepository,
@@ -46,5 +46,5 @@ object BuildServices {
 
 
 private
-val StartParameter.isKotlinDslBuildCacheEnabled: Boolean
-    get() = projectProperties.getOrDefault("org.gradle.kotlin.dsl.caching.buildcache", null) == "true"
+val isKotlinDslBuildCacheEnabled: Boolean
+    get() = System.getProperty("org.gradle.kotlin.dsl.caching.buildcache", null) == "true"

subprojects/provider/src/test/kotlin/org/gradle/kotlin/dsl/caching/AbstractScriptCachingIntegrationTest.kt

@@ -45,7 +45,15 @@ abstract class AbstractScriptCachingIntegrationTest : AbstractIntegrationTest()
 
     protected
     fun buildWithUniqueGradleHome(vararg arguments: String): BuildResult =
-        buildForCacheInspection("-g", uniqueGradleHome(), *arguments)
+        buildWithGradleHome(uniqueGradleHome(), *arguments)
+
+    protected
+    fun buildWithGradleHome(gradleHomePath: String, vararg arguments: String) =
+        buildForCacheInspection("-g", gradleHomePath, *arguments)
+
+    protected
+    fun <T> withUniqueGradleHome(f: (String) -> T): T =
+        f(uniqueGradleHome())
 
     private
     fun uniqueGradleHome() =

subprojects/provider/src/test/kotlin/org/gradle/kotlin/dsl/caching/BuildCacheIntegrationTest.kt

@@ -59,7 +59,7 @@ class BuildCacheIntegrationTest : AbstractScriptCachingIntegrationTest() {
 
     @LeaksFileHandles("on the separate Gradle homes")
     @Test
-    fun `build cache integration is enabled via project property`() {
+    fun `build cache integration is enabled via system property`() {
 
         val buildCacheDir =
             existing("build-cache")
@@ -91,15 +91,22 @@ class BuildCacheIntegrationTest : AbstractScriptCachingIntegrationTest() {
             assertThat(output, containsString(expectedOutput))
         }
 
-        // Cache hit from build cache
-        buildWithUniqueGradleHome("--build-cache", withBuildCacheIntegration).apply {
+        // Cache hit from build cache (enabled via gradle.properties file)
+        withUniqueGradleHome { gradleHome ->
 
-            compilationCache {
-                misses(cachedSettingsFile)
-                hits(cachedBuildFile)
-            }
+            File(gradleHome, "gradle.properties").writeText(
+                "systemProp.$kotlinDslBuildCacheEnabled"
+            )
 
-            assertThat(output, containsString(expectedOutput))
+            buildWithGradleHome(gradleHome, "--build-cache").apply {
+
+                compilationCache {
+                    misses(cachedSettingsFile)
+                    hits(cachedBuildFile)
+                }
+
+                assertThat(output, containsString(expectedOutput))
+            }
         }
 
         // Cache miss without build cache integration
@@ -115,7 +122,10 @@ class BuildCacheIntegrationTest : AbstractScriptCachingIntegrationTest() {
     }
 
     private
-    val withBuildCacheIntegration = "-Porg.gradle.kotlin.dsl.caching.buildcache=true"
+    val kotlinDslBuildCacheEnabled = "org.gradle.kotlin.dsl.caching.buildcache=true"
+
+    private
+    val withBuildCacheIntegration = "-D$kotlinDslBuildCacheEnabled"
 
     private
     fun withLocalBuildCacheSettings(buildCacheDir: File): File =