Fix npm run build-backend

[endorpersand]

This script rebuilds the backend, but inconsistently saves it into node_modules if it already exists. You can tell because running npm run build-backend will result in a message along the lines of:

• up to date, audited 782 packages in 899ms (if lc3-backend is not resaved)
• changed 1 package, and audited 782 packages in 2s (if lc3-backend is resaved)

This is annoying for development, but doesn't affect the build process, because the GitHub Actions starts off with a fresh node_modules.

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Some solutions can be done to patch this for development:

1. Not using --install-links in the build script
2. Instead using pnpm i lc3-backend@../backend
3. Manually removing the package from node_modules before dev

(1) breaks builds because it symlinks lc3-backend into node_modules (which causes Electron Forge + Vite to freak)
(2) requires pnpm (and may also break builds(?))
(3) is annoying