Fix invalid CMake command in Linux building instructions

[Copilot]

The JavaScriptCore cmake invocation in BUILDING.md contained a stray -D flag (-D -D NAPI_JAVASCRIPT_ENGINE=...), making it an invalid command that would fail immediately.

Changes

• Remove stray -D from the JavaScriptCore cmake command
• Add -B build/linux to both JavaScriptCore and V8 cmake commands — consistent with every other platform section in the docs
• Replace bare ninja with cmake --build build/linux to match the out-of-source build pattern used elsewhere

Before/after for the broken command:

# Before (invalid)
cmake -G Ninja -D -D NAPI_JAVASCRIPT_ENGINE=JavaScriptCore

# After
cmake -B build/linux -G Ninja -D NAPI_JAVASCRIPT_ENGINE=JavaScriptCore

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