Update rest of build func

[cricalix]

✍️ Description

In #1202, I proposed that a new environment variable could be used to change the URL that the scripts fetch data from, enabling easy testing against custom repositories (or even local disk) without having to edit the files every time. In #1230, the request was made to split the PR into smaller PRs.

This PR implements the remaining changes to the build.func file, replacing static URLs with variable-based URLs.

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• Related PR: [overrideable url] Remaining changes to ct scripts to use environment variable #1252
• Related Discussion: RFC: Make the ProxmoxVE URLs "relocatable" with an environment variable #1202

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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)

Previous PR screenshots apply.