ci(digest): Add digest pinning

[merkata]

Applicable spec:

Overview

Adds digest pinning to rockfiles.

Rationale

Adds digest pinning to rockfiles.

Juju Events Changes

n/a

Module Changes

n/a

Library Changes

n/a

Checklist

• [ x] The charm style guide was applied
• [ x] The contributing guide was applied
• [ x] The changes are compliant with ISD054 - Managing Charm Complexity
• [ x] The documentation is generated using src-docs
• [ x] The documentation for charmhub is updated
• [ x] The PR is tagged with appropriate label (urgent, trivial, complex)