Fix license permissions to be removable

This fix will ensure that all files added to _build/licenses have permission 644 for files and 755 for directories. This is needed due to an issue during notice generation.

The go-licenses tool is used to generate a notice from all of Gitaly's Go-based dependencies. Sometimes the entire source code is copied as a notice when the license is unknown. This is the case specifically with Hashicorp's go-uuid library. When copied, it retains the permissions 444 for files and 555 for directories. This prevents removal when the notice is regenerated.