spectrum: kill a cava left by an earlier shell on start

stopping the shell leaves its cava running (parent 1), reading on from a
closed pipe and ignoring SIGTERM; every restart while music played leaked
one. on start, pkill -KILL -f our cava.conf before starting our own

src/spectrum.rs

@@ -35,6 +35,12 @@ impl Service for Spectrum {
     fn listen() {
         let config = art::folder().join("cava.conf");
 
+        /*
+         * a cava left by an earlier shell: stopping the shell doesn't stop its cava,
+         * which reads on from a closed pipe and ignores SIGTERM. none of ours runs yet
+         */
+        let _ = Command::new("pkill").arg("-KILL").arg("-f").arg(&config).status();
+
         let _ = fs::write(&config, cava_config());
 
         let mut running: Option<Capture> = None;