Use stable rustfmt

And install it as the container doesn't have it. Fortunately it's rather
quick (few seconds).

The previous nightly container apparently dropped rustfmt, which is
understandable as not all tools are available all the time. That's on
us.

Signed-off-by: Maxime “pep” Buquet <pep@bouah.net>

.gitlab-ci.yml

@@ -26,9 +26,11 @@ variables:
 
 rustfmt:
   stage: lint
-  image: rustlang/rust:nightly
   script:
+    - rustup component add rustfmt
     - find -type f -iname '*.rs' -not -path './target/*' | xargs rustfmt --check --edition 2018
+  extends:
+    - .stable
 
 rust-latest-build:
   extends: