fix: Windows ARM64 update architecture detection

[Vinson6688-lee]

Description

Fixes issue #141031 where Windows ARM64 devices running x64 VS Code under emulation receive the wrong architecture update.

Problem

On Windows ARM64 devices (e.g., Surface Pro X), when running x64 VS Code under emulation:

• process.arch returns 'x64' (the process architecture)
• But the OS is actually ARM64
• The update service downloads x64 updates instead of arm64

Solution

Use os.arch() instead of process.arch to detect the actual OS architecture, ensuring ARM64 devices receive the correct arm64 build regardless of the running process architecture.

Changes

• Added getOSArch() helper function to normalize architecture detection
• Updated cachePath getter to use getOSArch()
• Updated buildUpdateFeedUrl() to use getOSArch()

Testing

This fix ensures that:

• ARM64 devices get arm64 updates (even when running x64 VS Code)
• x64 devices continue to get x64 updates
• ia32 devices continue to get ia32 updates

Fixes #141031

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