`buffrs::build` should be resilient to non-existent credentials

[mara-schulke]

Currently we open the credential file eagerly and expecting it to be present. buffrs::build should not error if it does not use the registry (and thus require the credentials)

So we should move from

1. Open the credential file
2. Check if protos are missing
3. Download (eventually) missing protos
4. Compile protos

To

1. Check if protos are missing
2. If yes, open credential file and download protos
3. Compile Protos

[mara-schulke]

FYI @tetigi

[tetigi]

Current workaround requires running buffrs login to generate the credentials file.