fix(tracing): Don't finish React Router 6 pageload transactions early

[onurtemizkan]

Fixes: #6470

Stops finishing active transactions in the second useEffect call (first one only runs on pageload) for pageload transactions.

Instead, skips the second useEffect altogether when the first one runs.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.78 KB (-0.02% 🔽)
@sentry/browser - ES5 CDN Bundle (minified)	61.27 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.57 KB (+0.02% 🔺)
@sentry/browser - ES6 CDN Bundle (minified)	54.8 KB (0%)
@sentry/browser - Webpack (gzipped + minified)	20.34 KB (0%)
@sentry/browser - Webpack (minified)	66.48 KB (0%)
@sentry/react - Webpack (gzipped + minified)	20.36 KB (0%)
@sentry/nextjs Client - Webpack (gzipped + minified)	47.56 KB (0%)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	26.75 KB (0%)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	25.19 KB (-0.01% 🔽)
@sentry/replay ES6 CDN Bundle (gzipped + minified)	42.08 KB (-0.01% 🔽)
@sentry/replay - Webpack (gzipped + minified)	38.17 KB (0%)

[timfish]

Looks good although I lacked the context to fix this myself so probably not the best person to judge!

Maybe add a test to ensure this doesn't regress?

[onurtemizkan]

Closing in favour of #6609