Screenshots of tests are saved in configuration-specific subdirectory

Since XWIKI-24178, when integration tests are executed with "maven.build.dir" set, screenshots are saved in a subdirectory that depends on the configuration name. So with our CI setup for environment tests, we get something like target/mysql-latest-pom-tomcat-11-jdk25-filesystem-chrome/mysql-latest-default-tomcat-11-jdk25-chrome/screenshots/ which is harder to detect for CI. Instead, we should save screenshots simply in a screenshots subdirectory of the Maven build directory.