Defer testOnlyFiles via Object.toString rather than Provider<String>.

The latter caused the env var value to be wrapped in "fixed(...)" for some reason.

Author: prdoyle

build-tools-internal/src/main/java/org/elasticsearch/gradle/internal/ElasticsearchTestBasePlugin.java

@@ -182,7 +182,12 @@ public void execute(Task t) {
                 FileCollection mainRuntime = mainSourceSet.getRuntimeClasspath();
                 FileCollection testRuntime = testSourceSet.getRuntimeClasspath();
                 FileCollection testOnlyFiles = testRuntime.minus(mainRuntime);
-                test.environment("es.entitlement.testOnlyPath", project.provider(testOnlyFiles::getAsPath));
+                test.environment("es.entitlement.testOnlyPath", new Object() {
+                    @Override
+                    public String toString() {
+                        return testOnlyFiles.getAsPath();
+                    }
+                });
             }
 
             test.systemProperties(getProviderFactory().systemPropertiesPrefixedBy("tests.").get());