ci: fix missing tag variable in release note generation

[yoshikawa]

What I did

• Added tag="${GITHUB_REF##*/}" to the Prepare the release note step in .github/workflows/release.yaml.

Why

At the time of the v2.3.5 release, the generated release note contained an empty tag placeholder in the download URL, e.g.: curl -sSL https://github.com/rootless-containers/rootlesskit/releases/download//rootlesskit-$(uname -m).tar.gz

This change ensures that ${tag} is correctly set and expands to the current release tag (e.g., v3.0.0), so that the release note contains a valid download URL.

Example

After this fix, the generated note will correctly include the tag: curl -sSL https://github.com/rootless-containers/rootlesskit/releases/download/v3.0.0/rootlesskit-$(uname -m).tar.gz | tar Cxzv ~/bin

QA

Verified in my own sandbox repository: https://github.com/yoshikawa/sandbox/releases/tag/untagged-e0f32bf5925c75582d17

Steps to reproduce:

git tag -a v0.0.7 -m "v0.0.7" 5d93db9
git push origin v0.0.7
# Verified the GitHub Actions run:
# https://github.com/yoshikawa/sandbox/actions/runs/19010835989/job/54291534935#step:7:28

[yoshikawa]

📝FYI

It looks like the line tag="${GITHUB_REF##*/}" was removed in #394 , which causes the tag variable to be unset again during release note generation.
As a result, the tag is missing in the generated download URL (e.g., .../releases/download//rootlesskit-$(uname -m).tar.gz).