fix(dev): Restrict size check action to PRs

[lobsterkatie]

The size check action we use in our main CI workflow will throw an error if run outside of the context of a PR (as when running CI manually, for example). As a result, the overall run is always marked as a failure, even when all other jobs passed. This restricts the action to running on PRs, to avoid that problem.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	18.78 KB (-6.77% 🔽)
@sentry/browser - ES5 CDN Bundle (minified)	58.38 KB (-9.65% 🔽)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	17.54 KB (-6.99% 🔽)
@sentry/browser - ES6 CDN Bundle (minified)	52.61 KB (-9.26% 🔽)
@sentry/browser - Webpack (gzipped + minified)	19.33 KB (-16.83% 🔽)
@sentry/browser - Webpack (minified)	61.47 KB (-24.77% 🔽)
@sentry/react - Webpack (gzipped + minified)	19.35 KB (-16.87% 🔽)
@sentry/nextjs Client - Webpack (gzipped + minified)	42.81 KB (-10.91% 🔽)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	24.43 KB (-6.3% 🔽)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	23 KB (-6.05% 🔽)

[lforst]

I wonder if this will mess with the "comparing" functionality because the action won't trigger on a push to master.

[lobsterkatie]

I wonder if this will mess with the "comparing" functionality because the action won't trigger on a push to master.

I think that still counts as a PR - it must, because the action explicitly only works on pull_request events.