Use custom buildFileName in subprojects

Use a custom buildFileName in each sub-project of the multi-project Gradle build. See https://docs.gradle.org/current/userguide/fine_tuning_project_layout.html#sub:modifying_element_of_the_project_tree

This is needed to test gitlab-org/security-products/analyzers/gemnasium-maven!131 (merged), which fixes one of the two problems captured in gitlab-org/gitlab#323921:

in a multi-project build when the parent project has a build.gradle, sub-projects aren't properly scanned if their build files aren't named build.gradle; this was reported in gitlab-org/gitlab#322346 (closed)

The problem has is confirmed in https://gitlab.com/gitlab-org/security-products/tests/java-gradle-multimodules/-/jobs/1747509658 (latest release of gemnasium-maven): subprojects are skipped.

This scanning job proves that the aforementioned MR fixes the problem (though the package manager don't match in the report, but that's not related): https://gitlab.com/gitlab-org/security-products/tests/java-gradle-multimodules/-/jobs/1747525741