[BRE-2024] Fix x86_64 macOS NAPI build producing ARM64 binaries (#1531)

## 🎟️ Tracking

https://bitwarden.atlassian.net/browse/BRE-2024

## 📔 Objective

When [BRE-1302](https://bitwarden.atlassian.net/browse/BRE-1302)
upgraded the macOS runner from `macos-13` (Intel) to `macos-15` (ARM64),
the `x86_64` build job began running on ARM64 hardware. The build
command `npm run build` uses the `--platform` flag, which auto-detects
the host architecture rather than respecting the target triple. This
caused the `x86_64` job to build `ARM64` binaries instead.

Changes:
- Add `build-x64` script to `package.json` that explicitly targets
`x86_64-apple-darwin`, mirroring the existing `build-arm64` pattern
- Update `build-napi.yml` to use `npm run build-x64` for the `x86_64`
job

This ensures the `x86_64 job` produces `sdk-napi.darwin-x64.node`
(Intel) instead of incorrectly producing `sdk-napi.darwin-arm64.node`
(ARM).

## 🚨 Breaking Changes

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[BRE-1302]:
https://bitwarden.atlassian.net/browse/BRE-1302?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ