[GenFacades] Autogenerated PNSE stubs are not regenerated after ApiExclusionListPath file changes

- [ ] This issue is blocking
- [x] This issue is causing ~unreasonable~ unnecessary pain

Noticed when working on https://github.com/dotnet/runtime/issues/65802, when I edit the contents of the file pointed to by the `ApiExclusionListPath` property, the file is not regenerated during the build. The autogenerated file needs to be manually deleted in order for the changes to propagate to the next build.

Since there is no feedback whatsoever (e.g. no error if the file does not exist), I didn't know why adding the API exclusion list did not apply and it took me an unreasonable time to find out.