Vendordep update action doesn't work with multiple files

[spacey-sooty]

To reproduce:

1. add the Phoenix vendordep to testing/cpp/vendordeps at beta-2
2. run ./gradlew vendordep --update
3. See Phoenix updated to beta-3
4. revert the update
5. add choreolib2025beta to testing/cpp/vendordeps
6. run ./gradlew vendordep --update
7. see Phoenix isnt updated