only install if not already installed

Author: japaric

ci/install.sh

@@ -3,8 +3,8 @@ set -ex
 main() {
     case $TARGET in
         thumbv*-none-eabi*)
-            cargo install xargo || true
-            rustup component add rust-src || true
+            cargo install --list | grep xargo || cargo install xargo
+            rustup component list | grep rust-src || rustup component add rust-src
             ;;
     esac
 }