chore(build): Only upload lambda layer when releasing

[lobsterkatie]

Note: This is an exact duplicate of #5167, which accidentally got merged into the 7.x branch rather than master.

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)	19.43 KB (-3.53% 🔽)
@sentry/browser - ES5 CDN Bundle (minified)	60.2 KB (-6.83% 🔽)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.23 KB (-3.32% 🔽)
@sentry/browser - ES6 CDN Bundle (minified)	53.82 KB (-7.15% 🔽)
@sentry/browser - Webpack (gzipped + minified)	19.99 KB (-13.96% 🔽)
@sentry/browser - Webpack (minified)	65.19 KB (-20.22% 🔽)
@sentry/react - Webpack (gzipped + minified)	20.02 KB (-14.01% 🔽)
@sentry/nextjs Client - Webpack (gzipped + minified)	43.76 KB (-8.93% 🔽)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	25.46 KB (-2.38% 🔽)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	23.98 KB (-2.05% 🔽)