Fix: Rules can't be empty: Just always run the build job

.gitlab-ci.yml

@@ -90,7 +90,8 @@ build:cargo:x86_64-unknown-linux-gnu:
   extends: .build-cargo-shared-settings
   variables:
     TARGET: "x86_64-unknown-linux-gnu"
-  rules: []
+  rules:
+    - if: "$CI_COMMIT_BRANCH"
 
 build:cargo:armv7-unknown-linux-gnueabihf:
   extends: .build-cargo-shared-settings