Fixed output name makes it impossible to deploy two pipelines to the same account and region

[donatoaz]

https://github.com/aws/aws-prototyping-sdk/blob/81cde33a61a32a1d80aaf867acb71bc56a3ecfc6/packages/pipeline/src/pdk-pipeline.ts#L172

The export name should be prefixable by a parameter in a way as to permit multiple deployments to the same account/region.

As it is today, trying to deploy a second PDKPipeline will fail with the error:

Export with name CodeRepositoryGRCUrl is already exported by stack PipelineStack. Rollback requested by user.

[cogwirrel]

Thanks for pointing this out! Agreed it would be nice not to hardcode the export name! Perhaps we could prefix with the repositoryName since that's already an input that would need to be unique per PdkPipeline? :)

I've opened this as an issue here: #239