project: use non-nightly image by default in CI

Only use the nightly features for `cargo fmt`. This avoids having the CI
be red because `clippy` is not available for the nightly version that
was used to run it.

.gitlab-ci.yml

@@ -1,5 +1,5 @@
 # Official language image.
-image: 'rustlang/rust:nightly'
+image: 'rust:latest'
 
 stages:
   - lint
@@ -10,6 +10,8 @@ variables:
   APT_CACHE_DIR: $CI_PROJECT_DIR/apt
 
 lint:fmt:
+  # Nightly image for formatting configuration
+  image: 'rustlang/rust:nightly'
   stage: lint
   before_script:
     - rustup component add rustfmt