Use unqualified path to bundler

So users can run it locally when they have `bundler` installed
as debian package.

tests/platform-ci/setup.sh

 #!/bin/sh
 
 which bundle || /usr/bin/apt install bundle
-/usr/local/bin/bundle install --binstubs --path=vendor --with=test --jobs "$(nproc)"
-/usr/local/bin/bundle exec leap -v2 --yes help
+bundle install --binstubs --path=vendor --with=test --jobs "$(nproc)"
+bundle exec leap -v2 --yes help