ci: cancel superseded pipelines

Force-pushing a branch several times in a row leaves the older pipelines
running to completion against commits nobody is waiting on. Mark the job
interruptible so GitLab can reclaims the runners.

.gitlab-ci.yml

@@ -11,5 +11,6 @@ vlc-rs-build:
         - amd64
     stage: build
     image: $VLC_UBUNTU_IMAGE
+    interruptible: true
     script:
         - cargo build --all