ci : add nightly-tag.txt to make-release (#27485)

As agreed in ggml discussion #1579, the official semver releases now
include a nightly-tag.txt asset containing the tag of the corresponding
nightly release (e.g. b10485). The Web UI assets are published to the
HF bucket under the nightly tag, so this makes them discoverable for
each official release.

- make-release-desc.sh: expose the resolved nightly tag as a
  nightly_tag output
- make-release.yml: create nightly-tag.txt from that tag, upload it
  as a release asset (skipped on dry-run), mention it in the release
  body and in the dry-run summary

Assisted-by: pi:llama.cpp/Qwen3.8-27B

scripts/make-release-desc.sh

@@ -15,7 +15,8 @@
 # tag exists.
 #
 # Env (when running in GitHub Actions):
-#   GITHUB_OUTPUT: previous_tag, changelog_title, changelog and nightly are written here
+#   GITHUB_OUTPUT: previous_tag, changelog_title, changelog, nightly and nightly_tag
+#     are written here
 #   GITHUB_REPOSITORY: owner/repo, used to build the nightly release URL (skipped when unset)
 set -euo pipefail
 
@@ -80,6 +81,7 @@ if [[ -n "${GITHUB_OUTPUT:-}" ]]; then
         echo "previous_tag=${PREV}"
         echo "changelog_title=${CHANGELOG_TITLE}"
         echo "nightly=${NIGHTLY}"
+        echo "nightly_tag=${NIGHTLY_TAG}"
         echo "changelog<<CHANGELOG_EOF"
         echo "${CHANGELOG}"
         echo "CHANGELOG_EOF"