fix(nextjs): Fix automatic release value discovery

[lforst]

Fixes #6504

In we switched from the Sentry Webpack plugin to using a custom loader to inject the release value into the SDK that either reads from options or from environment variables. We didn't consider that the Webpack Plugin (or rather Sentry CLI) has a mechanism to determine a release value apart from options or environment variables.

This PR in essence reverts #6404 which introduced this bug.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.71 KB (-0.01% 🔽)
@sentry/browser - ES5 CDN Bundle (minified)	61.11 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.5 KB (-0.03% 🔽)
@sentry/browser - ES6 CDN Bundle (minified)	54.64 KB (0%)
@sentry/browser - Webpack (gzipped + minified)	20.28 KB (0%)
@sentry/browser - Webpack (minified)	66.33 KB (0%)
@sentry/react - Webpack (gzipped + minified)	20.3 KB (0%)
@sentry/nextjs Client - Webpack (gzipped + minified)	47.5 KB (+0.42% 🔺)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	26.7 KB (-0.02% 🔽)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	25.15 KB (+0.01% 🔺)
@sentry/replay ES6 CDN Bundle (gzipped + minified)	41.73 KB (-0.01% 🔽)
@sentry/replay - Webpack (gzipped + minified)	38 KB (0%)