Tag pre-releases as "nightly" rather than "latest"

"latest" is a bit special in a sense that Github will automatically
add that label to the latest final release. Let's avoid the name clash
and call it "nightly" instead.

Author: malor

.github/workflows/release.yml

@@ -31,7 +31,7 @@ jobs:
             echo "::set-output name=title::${GITHUB_REF#refs/tags/}"
           else
             echo "::set-output name=prerelease::true"
-            echo "::set-output name=release_tag::latest"
+            echo "::set-output name=release_tag::nightly"
             echo "::set-output name=title::Development Build"
           fi