Pull GitHub upload URL from the release created/being updated

[klaernie]

From reading the docs for uploading assets it seems that the upload_url is given in the response to either the create release request or the get release request, but currently a user needs to set it manually if it is not for github.com.

To me it seems it would be nice if goreleaser would never require manual configuration, but would simply use the upload URL determined by the release it created or is updating.

The URL is given in uritemplate(RFC6570) form - but there is a go library for this https://github.com/yosida95/uritemplate.

[caarlos0]

That's a upload URL for the release itself, I think.

The upload_url we accept in the config is the base upload URL.

[klaernie]

I dug a little deeper - it looks like the go-github implementation for UploadReleaseAsset() is implemented to only accept manually building the URL, never by following the hypermedia relation specified in the docs. I think I'll ask there to see if this can be changed.

But I at least learnt, that one can simply specify both the BaseURL and UploadURL as https://github.internal.domain/, and it will append the expected path components automatically (as long as the host component does not start with api. or contains .api.) - at least one will be able to after #6261 is merged