fix(build): Build bundles in parallel

[lobsterkatie]

This PR parallelizes our top-level build:bundle yarn script, in order to make it run faster. Because so much of the total time is spent on one package (@sentry/integrations, which currently produces 24 bundles to every other package's max 4 bundles), the time savings isn't earth-shattering, but it does run 20-30% faster in my informal tests.

[github-actions]

size-limit report

Path	Base Size (f715e87)	Current Size	Change
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.8 KB	19.8 KB	+0.01% 🔺
@sentry/browser - ES5 CDN Bundle (minified)	63.37 KB	63.37 KB	0%
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.44 KB	18.44 KB	-0.01% 🔽
@sentry/browser - ES6 CDN Bundle (minified)	56.41 KB	56.41 KB	0%
@sentry/browser - Webpack (gzipped + minified)	22.27 KB	22.27 KB	0%
@sentry/browser - Webpack (minified)	76.33 KB	76.33 KB	0%
@sentry/react - Webpack (gzipped + minified)	22.3 KB	22.3 KB	0%
@sentry/nextjs Client - Webpack (gzipped + minified)	46.44 KB	46.44 KB	0%
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	26.69 KB	26.68 KB	-0.01% 🔽