fix(build): repair VS Code package CI

[hongjr03]

Summary

• remove inherited CARGO_BUILD_TARGET when launching cargo xtask for Alpine VSIX packaging
• keep xtask as the single server-build entry point while preserving Alpine linker/RUSTFLAGS setup
• cover the env sanitization in package tests

Verification

• node --import tsx --test "test/package.test.ts" (in editors/vscode)
• npm run typecheck (in editors/vscode)