Retry pip user installs before breaking system packages

[Nacai (B67687)]

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This adjusts the pip retry flow for externally managed environments.

When pip reports externally-managed-environment, UniGetUI currently retries immediately with --break-system-packages. This change makes the first retry use --user instead, and only falls back to --break-system-packages if the user-scoped retry still cannot proceed.

That keeps the existing fallback in place, but prefers the less invasive pip path first.

Validation:

• Local source snapshot reviewed against the current main branch.
• I could not run dotnet build locally because the repository pins .NET SDK 10.0.103 in global.json, while this environment only has 6.0.428 installed.

Follow-up to #4542