[vector_graphics_compiler] Use Abi.current() for engine-artifact lookup on Linux ARM64

[HiroyukiTamura]

On Linux ARM64 hosts, vector_graphics_compiler fails with:

Could not locate libpathops at <flutter>/bin/cache/artifacts/engine/linux-x64/libpath_ops.so

Flutter ships the engine binary at .../engine/linux-arm64/libpath_ops.so, but
_initialize_path_ops_io.dart and _initialize_tessellator_io.dart hardcode
'linux-x64' on Linux and never look at the arm64 directory.

This change switches the Linux branch in both loaders to pick 'linux-arm64'
when Abi.current() == Abi.linuxArm64, and otherwise keeps the existing
'linux-x64' value. Behavior on windows-x64, darwin, and linux-x64 hosts is
unchanged. Bumps the package to 1.2.4 with a CHANGELOG entry.

Fixes flutter/flutter#158865.

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[gemini-code-assist]

Code Review

This pull request adds support for linux-arm64 hosts in the vector graphics compiler. It dynamically determines the platform directory for pathops and the tessellator loader using Abi.current() instead of hardcoding linux-x64. Additionally, the package version is bumped to 1.2.4 in pubspec.yaml and documented in CHANGELOG.md. There are no review comments to address.

[stuartmorgan-g]

[x] I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under.

Thanks for the contribution! Where is this item reflected in the PR?