test(nextjs): Disable doubleEndMethodOnVercel on Node 10

[AbhiPrasad]

Fixes #6784

Since doubleEndMethodOnVercel seems only to be flaky on Node 10, and it's very rare that NextJS apps are running on Node 10 (and impossible on Vercel), prevent this test from running in Node 10.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.84 KB (0%)
@sentry/browser - ES5 CDN Bundle (minified)	61.46 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.61 KB (+0.02% 🔺)
@sentry/browser - ES6 CDN Bundle (minified)	55 KB (0%)
@sentry/browser - Webpack (gzipped + minified)	20.37 KB (0%)
@sentry/browser - Webpack (minified)	66.54 KB (0%)
@sentry/react - Webpack (gzipped + minified)	20.4 KB (0%)
@sentry/nextjs Client - Webpack (gzipped + minified)	47.69 KB (0%)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	26.83 KB (+0.02% 🔺)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	25.28 KB (-0.01% 🔽)
@sentry/replay ES6 CDN Bundle (gzipped + minified)	43.43 KB (-0.83% 🔽)
@sentry/replay - Webpack (gzipped + minified)	38.65 KB (-0.96% 🔽)