This should make it easier to use the same config file with both oss and
pro versions when only the nightly feature is used, e.g. in #3501.
Currently, that would fail because `IsNightly` is not a template field,
so you won't really be able to do it without keeping a second
`.goreleaser.yml` file. With this change, `IsNightly` always eval to
false on OSS, and keeps working as before on Pro.
b1c8e9f25b