Always obtain fresh PropertySources in ConfigurableEnvironmentPropertySource

Although it's unlikely that the implementation of getPropertySources()
in a ConfigurableEnvironment would be overridden to return a different
MutablePropertySources instance than the one that the
ConfigurableEnvironment typically acts on, it is in fact possible.

In light of that possibility, this commit refactors
ConfigurableEnvironmentPropertySource so that it always obtains a fresh
PropertySources reference.

See gh-34861

spring-context/src/main/java/org/springframework/context/support/PropertySourcesPlaceholderConfigurer.java

@@ -227,19 +227,15 @@ public class PropertySourcesPlaceholderConfigurer extends PlaceholderConfigurerS
 	 */
 	private static class ConfigurableEnvironmentPropertySource extends PropertySource<ConfigurableEnvironment> {
 
-		private final PropertySources propertySources;
-
-
 		ConfigurableEnvironmentPropertySource(ConfigurableEnvironment environment) {
 			super(ENVIRONMENT_PROPERTIES_PROPERTY_SOURCE_NAME, environment);
-			this.propertySources = environment.getPropertySources();
 		}
 
 
 		@Override
 		@Nullable
 		public Object getProperty(String name) {
-			for (PropertySource<?> propertySource : this.propertySources) {
+			for (PropertySource<?> propertySource : super.source.getPropertySources()) {
 				Object candidate = propertySource.getProperty(name);
 				if (candidate != null) {
 					return candidate;
@@ -250,7 +246,7 @@ public class PropertySourcesPlaceholderConfigurer extends PlaceholderConfigurerS
 
 		@Override
 		public boolean containsProperty(String name) {
-			for (PropertySource<?> propertySource : this.propertySources) {
+			for (PropertySource<?> propertySource : super.source.getPropertySources()) {
 				if (propertySource.containsProperty(name)) {
 					return true;
 				}
@@ -260,7 +256,7 @@ public class PropertySourcesPlaceholderConfigurer extends PlaceholderConfigurerS
 
 		@Override
 		public String toString() {
-			return "ConfigurableEnvironmentPropertySource {propertySources=" + this.propertySources + "}";
+			return "ConfigurableEnvironmentPropertySource {propertySources=" + super.source.getPropertySources() + "}";
 		}
 	}