hide the thumbv* target files

due to how Cargo works, the rustc-cfg in build.rs was picking our target
specifications rather than the one the user meant to use. Placing our
target files in any place other than the root of the Cargo project
avoids the issue.

ci/run.sh

@@ -3,6 +3,8 @@ set -e
 # Test our implementation
 case $1 in
     thumb*)
+        export RUST_TARGET_PATH=/checkout/targets
+
         xargo build --target $1
         xargo build --target $1 --release
         ;;