[core] build.func&install.func: Fix ssh keynot added error

[dsiebel]

✍️ Description

A PR I created for develop was recently cherry-picked into main (#1456), but it only contained one half of the changes.
The SSH key that can be specified during advanced setup was not being added to the container.
This PR addresses that, recovering the changes made in the original PR.

• Related PR: feat: allow adding SSH authorized key for root (advanced settings) by @dsiebel #1456
• Related PR: feat: allow adding SSH authorized key for root (advanced settings) #1060
• Related Discussion: Adding ssh key based authentication as a way to connect #860

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🛠️ Type of Change

Please check the relevant options:

• Bug fix (non-breaking change that resolves an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change unexpectedly)
• New script (a fully functional and thoroughly tested script or set of scripts)

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✅ Prerequisites

The following steps must be completed for the pull request to be considered:

• Self-review performed (I have reviewed my code to ensure it follows established patterns and conventions.)
• Testing performed (I have thoroughly tested my changes and verified expected functionality.)
• Documentation updated (I have updated any relevant documentation)

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📋 Additional Information (optional)

When creating the original PR, I was also pondering the idea of skipping the part that replaces

PermitRootLogin prohibit-password`

with

PermitRootLogin yes

in the sshd config, when a root password has been set.
What do you think, would this be a worthy improvement?