Fix "build" source folder being ignored (Gradle)

[niklaswimmer]

Reasons for making this change:

I used the template and my files in buildSrc/src/**/build/ (my package name includes build) were being ignored. I tried around and found out that with this change they are no longer ignored. This was originally introduced in #3370, but the previous configuration only works if src is a top level directory.

Links to documentation supporting these rule changes:

If there is a separator at the beginning or middle (or both) of the pattern, then the pattern is relative to the directory level of the particular .gitignore file itself.

From https://git-scm.com/docs/gitignore, emphasizes mine.