ref(build): Move build directory to main rollup config

[lobsterkatie]

While reviewing #4688, it occurred to me that our bundles directory, build, is the same in every package, and therefore should live in the centralized rollup config rather than all of the package-specific ones.

[github-actions]

size-limit report

Path	Base Size (af7081c)	Current Size	Change
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.37 KB	19.37 KB	-0.01% 🔽
@sentry/browser - ES5 CDN Bundle (minified)	61.92 KB	61.92 KB	0%
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.04 KB	18.04 KB	+0.02% 🔺
@sentry/browser - ES6 CDN Bundle (minified)	55.32 KB	55.32 KB	0%
@sentry/browser - Webpack (gzipped + minified)	22.3 KB	22.3 KB	0%
@sentry/browser - Webpack (minified)	76.58 KB	76.58 KB	0%
@sentry/react - Webpack (gzipped + minified)	22.33 KB	22.33 KB	0%
@sentry/nextjs Client - Webpack (gzipped + minified)	46.56 KB	46.56 KB	0%
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	25.25 KB	25.25 KB	0%
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	23.65 KB	23.65 KB	0%

[Lms24]

These changes make a lot of sense to me. Looking good.