build: Reimagine linker warnings in different scenarios

[mathbunnyru]

High Level Overview of Change

(macOS + Linux, CI) Warnings will be fatal - this would allow us to catch problem.
(macOS, locally) Try to disable specific warnings about deployment_target_mismatches, in Nix silence all (there is no separate option)

Context of Change

API Impact

• Public API: New feature (new methods and/or new fields)
• Public API: Breaking change (in general, breaking changes should only impact the next api_version)
• libxrpl change (any change that may affect libxrpl or dependents of libxrpl)
• Peer protocol change (must be backward compatible or bump the peer protocol version)

[Copilot]

Pull request overview

This PR adjusts how the build treats linker warnings across environments: making linker warnings fatal in CI on macOS/Linux to catch real issues early, while reducing local macOS noise from deployment target mismatch warnings (with special handling for the Nix toolchain).

Changes:

• In CI on macOS/Linux, add linker flags to treat all linker warnings as errors (-fatal_warnings on macOS, --fatal-warnings on Linux).
• On local macOS, suppress deployment target mismatch warnings; under Nix, silence all linker warnings due to limited per-warning control.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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