This PR introduces an environment variable that allows to modify the `EdgeRuntime` value on compilation time.
This is done to allow cloud providers like Vercel to have a different value, and enable user code and 3rd party libraries to have different code paths depending on the Edge Functions provider.
## Related
- Related to #30739
## Bug
- [x] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`