Gradle: Enable JaCoCo only if a JacocoReport task is in the graph

Enabling JaCoCo with "append = true" (the default) disables Gradle's
build cache. Instead of setting "append = false", enable JaCoCo only if
we actually request a coverage report. For details see

https://discuss.gradle.org/t/do-not-cache-if-condition-matched-jacoco-agent-configured-with-append-true-satisfied/23504

build.gradle

@@ -97,10 +97,6 @@ subprojects {
         }
     }
 
-    jacoco {
-        toolVersion = '0.8.0'
-    }
-
     test {
         testLogging {
             events 'started', 'passed', 'skipped', 'failed'
@@ -130,6 +126,17 @@ subprojects {
         }
     }
 
+    jacoco {
+        toolVersion = '0.8.0'
+    }
+
+    gradle.taskGraph.whenReady { graph ->
+        def enabled = graph.allTasks.any { it instanceof JacocoReport }
+        tasks.withType(Test) {
+            jacoco.enabled = enabled
+        }
+    }
+
     task funTestCoverage(type: JacocoReport, dependsOn: funTest) {
         description = 'Generates the code coverage report for the functional tests.'
         group = 'Reporting'