feat(ci): Cache node_modules in flaky test detector

[billyvg]

This should cut down on the yarn install step.

Take a look at these two runs:

Uncached - 4s setup node + 3m8s to yarn install (2m 31s post setup node to save cache)
Cached - 42s setup node + 1m42s to yarn install

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	75.07 KB (+0.29% 🔺)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	31.11 KB (0%)
@sentry/browser - Webpack (gzipped)	21.81 KB (0%)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	69.67 KB (+0.19% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	28.16 KB (+0.04% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped)	20.18 KB (+0.05% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	219.68 KB (+0.28% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	84.69 KB (+0.03% 🔺)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	59.86 KB (+0.04% 🔺)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	31.03 KB (+0.03% 🔺)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	65.27 KB (+0.34% 🔺)
@sentry/react - Webpack (gzipped)	21.84 KB (0%)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	92.89 KB (+0.27% 🔺)
@sentry/nextjs Client - Webpack (gzipped)	50.64 KB (+0.06% 🔺)