There will not be nightly builds. ServiceStack v3 (BSD) will be released on a monthly basis to NuGet, as noted here. So compiling your own version from GitHub in the interim period between NuGet releases will have to do.
Service Stack still hosts continuous integration builds and publishes monthly periodic releases of ServiceStack v3 to NuGet.
I am not sure where you are seeing lots of fixes. I note a commit a month ago on the v3 branch, and a minor changes on the v3-fixes branch. So 3.9.71 is still pretty fresh.
I wouldn't expect as many commits to v3 for it to be necessary to do nightly builds, as a lot of effort is being concentrated into v4.