Migrate message_ix workflows

[khaeru]

This PR migrates the "lint" and "publish" workflows from message_ix to be reused across multiple repos; see “Reusing workflows” in the GitHub docs.

The workflow was prepared using the following, per this example:

pip install git-filter-repo

gh repo clone iiasa/actions
gh repo clone iiasa/message_ix

# Source repo
cd message_ix

# Filter only commits affecting these files
git filter-repo --path .github/workflows/lint.yaml --path .github/workflows/publish.yaml

# Target repo
cd ../actions

# New branch to which to add the filtered history
git checkout -b migrate-message-ix-workflows

# Merge the filtered history into the branch
git remote add tmp ../message_ix/
git fetch tmp
git merge remotes/tmp/main --allow-unrelated-histories