only re-run CI if necessary

.gitlab-ci.yml

@@ -24,17 +24,26 @@ stages:
   stage: build
   parallel:
     matrix:
-      - FEATURES: [ 'xdg-mime-backend', 'infer-backend', 'multi-threaded xdg-mime-backend',
+      - FEATURES:
-                    'multi-threaded infer-backend' ]
+        - xdg-mime-backend
+        - infer-backend
+        - multi-threaded xdg-mime-backend
+        - multi-threaded infer-backend
+
+  only:
+    refs:
+      - branches
+      - tags
+    changes:
+      - "/src/**/*"
+      - .gitlab-ci.yml
+
   script:
     cargo build --no-default-features --locked --features="$FEATURES"
 
 .cargo-test:
+  extends: .cargo-build
   stage: test
-  parallel:
-    matrix:
-      - FEATURES: [ 'xdg-mime-backend', 'infer-backend', 'multi-threaded xdg-mime-backend',
-                    'multi-threaded infer-backend' ]
   script:
     cargo test --no-default-features --locked --verbose --features="$FEATURES"