function copyAssetToBuild( filename, content = null, addSuffix = true ) {
	let destFilename = filename;
	if ( addSuffix ) {
		destFilename = destFilename
			.replace( /(\.[^.]+)$/, '-' + assetCacheBuster + '$1' );
	}
	const destPath = path.join( siteBuildPath, 'assets', destFilename );
	if ( ! content ) {
		const srcPath = path.join( sitePath, 'assets', filename );
		content = fs.readFileSync( srcPath );
	}
	fs.writeFileSync( destPath, content );
	return '/assets/' + destFilename;
}
Write a file to site/build/assets/ (from memory or from an existing file in
site/assets/) and include a cache buster in the new name.  Return the URL to
the asset file.