Speed up workflow validation and releases

[marcleblanc2]

Summary

• avoid duplicate package builds during release validation
• download pinned actionlint binary instead of building from source
• use cache action v5 and remove redundant pip upgrades
• let release validation and build jobs run in parallel before publishing

Test plan

• /opt/homebrew/bin/actionlint
• git diff --check -- .github/workflows/release.yml .github/workflows/validate.yml