Fix apt CLI stability warning by replacing apt with apt-get in installdependencies.sh

[Copilot]

This PR resolves the warning "WARNING: apt does not have a stable CLI interface. Use with caution in scripts." that appears when running the Azure Pipelines Agent installation dependencies script on Debian-based systems.

Problem

The installdependencies.sh script was using the apt command, which is designed for interactive use and generates warnings when used in scripts:

WARNING: apt does not have a stable CLI interface. Use with caution in scripts.

Solution

Replaced all apt commands with apt-get equivalents, which provides a stable CLI interface specifically designed for scripting:

• Before: apt update && apt install -y <packages>
• After: apt-get update && apt-get install -y <packages>

Changes Made

1. Updated comment from "prefer apt over apt-get" to "use apt-get for stable CLI interface in scripts"
2. Changed package manager detection from command -v apt to command -v apt-get
3. Replaced all apt update commands with apt-get update
4. Replaced all apt install commands with apt-get install
5. Updated error messages to reference apt-get instead of apt
6. Simplified logic by removing the fallback to apt (since apt-get is always available when apt is)

Benefits

• ✅ Eliminates the CLI stability warning completely
• ✅ Uses the stable, script-friendly interface consistently
• ✅ Maintains all existing functionality
• ✅ Reduces code complexity by removing redundant fallback logic
• ✅ More robust for automation scenarios

Testing

• Script syntax validation passes
• No remaining apt commands (only apt-get)
• All error messages updated consistently
• Integration test confirms warnings are eliminated

The fix is minimal and surgical, addressing the root cause while maintaining full backward compatibility.

Fixes #5149.

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