ci: Use lockfile as cache key

This ensures we don't keep stale builds of dependencies around.

.gitlab-ci.yml

@@ -1,13 +1,22 @@
 variables:
   CARGO_HOME: $CI_PROJECT_DIR/cargo
 
-cache:
-  paths:
-    - $CARGO_HOME
-    - target/
+# Default cache for Rust jobs - keyed on Cargo.lock to prune stale dependencies
+.rust_cache: &rust_cache
+  cache:
+    key:
+      files:
+        - Cargo.lock
+    paths:
+      - $CARGO_HOME/bin/
+      - $CARGO_HOME/registry/index/
+      - $CARGO_HOME/registry/cache/
+      - $CARGO_HOME/git/db/
+      - target/
 
 build-release:
   image: rust:latest
+  <<: *rust_cache
   script:
     - cargo build --verbose --release
   artifacts:
@@ -17,17 +26,20 @@ build-release:
 
 test:
   image: rust:latest
+  <<: *rust_cache
   script:
     - cargo test --verbose --release
 
 format_check:
   image: rust:latest
+  <<: *rust_cache
   script:
     - rustup component add rustfmt
     - cargo fmt -- --check
 
 clippy:
   image: rust:latest
+  <<: *rust_cache
   script:
     - rustup component add clippy
     - cargo clippy -- -D warnings