chore(build): Only upload lambda layer when releasing

[lobsterkatie]

This changes our GHA workflow to only zip and upload the lambda layer when the uploaded artifact is needed for a release. (This matches the behavior of other uploaded artifacts.)

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.36 KB (-3.88% 🔽)
@sentry/browser - ES5 CDN Bundle (minified)	60.05 KB (-7.07% 🔽)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.21 KB (-3.47% 🔽)
@sentry/browser - ES6 CDN Bundle (minified)	53.66 KB (-7.44% 🔽)
@sentry/browser - Webpack (gzipped + minified)	19.92 KB (-14.26% 🔽)
@sentry/browser - Webpack (minified)	63.12 KB (-22.76% 🔽)
@sentry/react - Webpack (gzipped + minified)	19.95 KB (-14.31% 🔽)
@sentry/nextjs Client - Webpack (gzipped + minified)	43.74 KB (-8.99% 🔽)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	25.38 KB (-2.67% 🔽)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	23.91 KB (-2.33% 🔽)