fix ci/install.sh

ci/install.sh

 set -euxo pipefail
 
 main() {
+    if [ $TARGET = thumbv7m-none-eabi ]; then
         local vers=0.3.7
 
         cargo install --list | grep "xargo v$vers" || \
-        cargo install xargo -f --vers $vers
+            ( cd .. && cargo install xargo -f --vers $vers )
 
         rustup component list | grep 'rust-src.*installed' || \
             rustup component add rust-src
+    fi
 }
 
 main