chore: Simplify yarn and alpine version update

[orgads]

Description

Before commit 3f6cb08 (fix: only update yarn if there are other changes to the dockerfile), yarnVersion depended on SKIP, so the substitute that followed made sense. This commit removed the condition and changed it to just substitute the version with itself, which doesn't make much sense.

Just remove it, and also move the real replace inside SKIP condition.

Motivation and Context

Testing Details

Tested yarn update.

Example Output(if appropriate)

Types of changes

• Documentation
• Version change (Update, remove or add more Node.js versions)
• Variant change (Update, remove or add more variants, or versions of variants)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Other (none of the above)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING.md document.
• All new and existing tests passed.

[Copilot]

Pull Request Overview

This PR streamlines the Dockerfile update script by removing redundant initial fetches of Alpine and Yarn versions and relocating the Yarn version lookup so it's only performed when updates are desired.

• Eliminated the early assignment of alpine_version and initial Yarn version fetch
• Removed rewriting the Dockerfile with its own current Yarn version
• Moved the real Yarn version fetch and substitution inside the SKIP check

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