ci/rustfmt: make sure to only check each file once

rustfmt has some magic that follows files (I'm guessing), making files get
checked multiple times with `*.rs`, so let's limit ourselves to `lib.rs`.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22321>

.gitlab-ci/test/gitlab-ci.yml

@@ -28,7 +28,7 @@ rustfmt:
     GIT_STRATEGY: fetch
   script:
     - shopt -s globstar
-    - rustfmt --check --verbose src/**/*.rs
+    - rustfmt --check --verbose src/**/lib.rs
 
 clang-format:
   # Cancel job if a newer commit is pushed to the same branch