Avoid having to pass remote to all `buf` commands for non-default registries

Feature

Right now, most buf commands default to buf.build as the registry and if a user is interacting with a different registry, they must pass their remote, even after having logged in through buf registry login. It would be nice if all buf commands, or at least the buf registry commands can interact with a default registry.

Some ways we can explore this feature:

• Use default key for .netrc (https://www.gnu.org/software/inetutils/manual/html_node/The-_002enetrc-file.html)
• Support some kind of BUF_REMOTE env var that can be set in managed environments