fix(nextjs): Use basePath for assetPrefix if needed

[lobsterkatie]

This fixes a bug in #6388, wherein basePath was ignored when determining assetPath. Nextjs copies the former over to the latter if the former is provided and the latter is not. In my original implementation of #6388, the SDK read the values after that had happened, so it was sufficient to look at asssetPath. When I moved the reading of the values earlier, I didn't account for the fact that basePath wouldn't have yet been copied over into assetPath, meaning we have to do it ourselves.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.6 KB (0%)
@sentry/browser - ES5 CDN Bundle (minified)	60.69 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.37 KB (+0.02% 🔺)
@sentry/browser - ES6 CDN Bundle (minified)	54.26 KB (0%)
@sentry/browser - Webpack (gzipped + minified)	20.13 KB (0%)
@sentry/browser - Webpack (minified)	65.83 KB (0%)
@sentry/react - Webpack (gzipped + minified)	20.16 KB (0%)
@sentry/nextjs Client - Webpack (gzipped + minified)	47.01 KB (+0.11% 🔺)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	26.44 KB (+0.01% 🔺)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	24.87 KB (+0.02% 🔺)
@sentry/replay ES6 CDN Bundle (gzipped + minified)	41.74 KB (+0.01% 🔺)
@sentry/replay - Webpack (gzipped + minified)	37.23 KB (0%)