Use side-effect free environment with tests rather than converting

Refine the logic introduced in 64270eca to use a side-effect free
Environment implementation rather than converting the Environment early.

Early conversion can cause condition evaluation issues if
`src/test/resources/application.properties` files are bound to the
`SpringApplication`. Specifically the `spring.main.web-application-type`
property can change the `Environment` type which must happen before
conditions are evaluated.

Fixes gh-29169

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/SpringApplication.java

@@ -389,7 +389,9 @@ public class SpringApplication {
 	 * @param environment the environment to convert
 	 * @return the converted environment
 	 * @since 2.5.7
+	 * @deprecated since 2.5.8 for removal in 2.7.0
 	 */
+	@Deprecated
 	public StandardEnvironment convertEnvironment(ConfigurableEnvironment environment) {
 		return new EnvironmentConverter(getClassLoader()).convertEnvironmentIfNecessary(environment,
 				deduceEnvironmentClass());