Get rid of warnings in bin/build

From https://github.com/handshake-org/handshake-org.github.io/actions/runs/4471682840/jobs/7856884549:
```
[DEPRECATED] The `--path` flag is deprecated because it relies on being remembered across bundler invocations, which bundler will no longer do in future versions. Instead please use `bundle config set --local path '../../build/vendor'`, and stop using this flag
```

Author: handshake-enthusiast

bin/build

@@ -50,5 +50,5 @@ echo "Finished."
 # build API documentation
 echo 'building api documentation...'
 cd ./src/api-docs
-bundle install --path=../../build/vendor
-bundle exec middleman build --clean --build-dir=../../build/api-docs
+BUNDLE_PATH="../../build/vendor" bundle install --jobs 4
+BUNDLE_PATH="../../build/vendor" bundle exec middleman build --clean --build-dir=../../build/api-docs