Use task outputs in distZip Paketo system tests

Using the outputs from `distZip` and `bootDistZip` as input to the
`bootBuildImage` task instead of indirectly using the output file
path prevents Gradle warnings about implicit task dependencies.

Fixes gh-32506

spring-boot-system-tests/spring-boot-image-tests/src/systemTest/resources/org/springframework/boot/image/paketo/PaketoBuilderTests-bootDistZipJarApp.gradle

@@ -37,5 +37,5 @@ application {
 }
 
 bootBuildImage {
-	archiveFile = new File("${buildDir}/distributions/${project.name}-boot.zip")
+	archiveFile = bootDistZip.archiveFile
 }

spring-boot-system-tests/spring-boot-image-tests/src/systemTest/resources/org/springframework/boot/image/paketo/PaketoBuilderTests-plainDistZipJarApp.gradle

@@ -37,5 +37,5 @@ application {
 }
 
 bootBuildImage {
-	archiveFile = new File("${buildDir}/distributions/${project.name}.zip")
+	archiveFile = distZip.archiveFile
 }