Allow disabling file-based keyring fallback

[mr-cal]

What needs to get done

• Add an environment variable SNAPCRAFT_DISABLE_FILE_BASED_KEYRING.
• Document how to disable the file-based keyring backend.

Why it needs to get done

Tools like ubuntu-image depend on keys created by snapcraft. If snapcraft uses a file-based backend, then other tools will have no access to those keys in the system keyring.

The user experience is confusing, because snapcraft list-keys will show keys that aren't accessible from the system keyring.

[syncronize-issues-to-jira]

Thank you for reporting us your feedback!

The internal ticket has been created: https://warthogs.atlassian.net/browse/CRAFT-3272.

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