Fix code coverage for nightly when run with the tests (#3667)

It was looking at whether the jars exist at the time that the
codeCoverageReport was being configured, rather than when it was being
executed. This now looks at the information lazily when actually
executing.

Author: ScottDugas

gradle/root.gradle

@@ -104,12 +104,11 @@ tasks.register('codeCoverageReport', JacocoReport) {
         "**/generated/*", "**/grpc/v1/**/*"
     ]
 
-    def compiledFiles = coverageSubprojects.collect { sp ->
-        new File(sp.buildDir, "libs/${sp.name}-${sp.version}.jar")
-    }.findAll { jf -> jf.exists() }
-    classDirectories.setFrom(compiledFiles.collect { jf ->
-        zipTree(jf).matching {
-            exclude exclusions
+    classDirectories.setFrom(coverageSubprojects.collect { sp ->
+        sp.layout.buildDirectory.file("libs/${sp.name}-${sp.version}.jar").map { jf ->
+            zipTree(jf).matching {
+                exclude exclusions
+            }
         }
     })