Use ClassLoader that will load Yaml to check if it's present

Fixes gh-27133

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/env/YamlPropertySourceLoader.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -43,7 +43,7 @@ public class YamlPropertySourceLoader implements PropertySourceLoader {
 
 	@Override
 	public List<PropertySource<?>> load(String name, Resource resource) throws IOException {
-		if (!ClassUtils.isPresent("org.yaml.snakeyaml.Yaml", null)) {
+		if (!ClassUtils.isPresent("org.yaml.snakeyaml.Yaml", getClass().getClassLoader())) {
 			throw new IllegalStateException(
 					"Attempted to load " + name + " but snakeyaml was not found on the classpath");
 		}