chore: encode gh actions outputs

[salasberryfin]

What this PR does / why we need it:

GitHub detects outputs containing substrings that are part of secrets as insecure and blocks us from sharing them with downstream jobs. This is a proposal to fix this issue by double encoding outputs so that they are obfuscated. Then, these outputs are decoded in the signing/provenance jobs.

These outputs are not actually sensitive data but just the image location and, worst case scenario (user decodes the string output), this data is public anyway.

Which issue(s) this PR fixes:
Fixes #211

Special notes for your reviewer:

Checklist:

• squashed commits into logical changes
• includes documentation
• adds unit tests
• adds or updates e2e tests

[Danil-Grigorev]

I think the better way to handle this is by using existing github tooling: https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#workflow

[salasberryfin]

I think the better way to handle this is by using existing github tooling: https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#workflow

The problem with this approach is that it uses an external secret store.