Support for GCB Executor

[frcamacho]

First off, thank you for developing such a great testing framework for Nextflow pipelines! It's been incredibly useful for ensuring pipeline quality.

I understand that executing nf-tests on the cloud is currently a limitation. However, I would love to explore the possibility of running nf-tests using cloud executors such as Google Cloud Build (GCB).

Even with down-sampling samples, running nf-test locally can be challenging due to resource limitations, particularly with large reference datasets. Leveraging cloud resources could significantly ease these constraints and enhance the testing process.