Fix the nginx cache snippets - I'd missed the file nesting somehow.
Tested on a debian:bookworm image with nginx-full installed, checked
that we could pull an arbitrary external site, as well as S3, as well
as GitLab artifacts.
Signed-off-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/34341>