BuildStream doesn't check that the artifact remotes has the required capabilities

Summary

As seen in https://gitlab.com/BuildStream/buildstream/-/jobs/327518918, when trying to connecting to remote artifact caches that are for bst1, BuildStream doesn't check the capabilities and skip them anymore

What is the current bug behavior?

BuildStream fails when a remote artifact cache is missing some capabilities

What is the expected correct behavior?

BuildStream should check for capabilities whenever trying a remote artifact cache