fix(nextjs): Use env variable for build detection

[lobsterkatie]

In non-Vercel-deployed nextjs apps using our SDK, we instrument the server in order to be able to capture transactions for a wider variety of incoming requests than is possible using our withSentry API route handler wrapper. We don't do this during build, however, because there's no server to wrap.

Until now, we've been relying on the fact that child processes of the main build process were invoked by node <path>/node_modules/jest-worker/build/workers/processChild.js in order to recognize them as such. Recently, however, next started vendoring pre-compiled versions of more of its dependencies, including jest-worker, such that now those child process are invoked by node <path>/node_modules/next/dist/compiled/jest-worker/processChild.js. This broke our build detection.

Rather than just switch to using the new path, this PR refactors the check to use an env variable set by the main build process, in order that the check not get fooled by any child processes spawned at runtime. (I don't actually know if that ever happens, but there's no reason it couldn't, so better to have a foolproof indicator.)

[github-actions]

size-limit report

Path	Base Size (8e4e671)	Current Size	Change
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.7 KB	19.7 KB	-0.01% 🔽
@sentry/browser - ES5 CDN Bundle (minified)	63.31 KB	63.31 KB	0%
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.37 KB	18.37 KB	-0.03% 🔽
@sentry/browser - ES6 CDN Bundle (minified)	56.48 KB	56.48 KB	0%
@sentry/browser - Webpack (gzipped + minified)	22.16 KB	22.16 KB	0%
@sentry/browser - Webpack (minified)	76.18 KB	76.18 KB	0%
@sentry/react - Webpack (gzipped + minified)	22.19 KB	22.19 KB	0%
@sentry/nextjs Client - Webpack (gzipped + minified)	46.33 KB	46.33 KB	0%
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	27.16 KB	27.15 KB	-0.01% 🔽

[AbhiPrasad]