Specifically, fe8bc3f23e made the "default" case `manual`
instead of `never`, which is causing all staging pipelines to have that
job regardless of any file `changes`, and always `manual` making it
block pipelines.
Let's make it automatic for upstream pipelines (ie. staging and
push-to-main), and only run on file `changes`.
Fixes: fe8bc3f23e ("ci: Only run rustfmt when necessary")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36260>
f065c2f0ac