Nightly updates correctly detected the newest version, but the installer
always queried GitHub’s `/releases/latest` endpoint. Since that endpoint
only returns the latest stable release, `iris update` downloaded the
stable binary even when the nightly channel was configured.
This change passes the selected nightly tag to the installer through
`IRIS_RELEASE_TAG`. When set, the installer queries
`/releases/tags/<tag>`, locates that nightly release’s assets, and
installs the correct binary. Stable updates continue to use
`/releases/latest` unchanged.
João Vitor Alves
0595ddd4b9