fix(ci): update release canary install path

[drew]

Summary

Update the release canary workflow to use an explicit install directory and add it to the GitHub Actions PATH. This keeps canary validation working with the new install.sh default install location.

Related Issue

N/A

Changes

• set OPENSHELL_INSTALL_DIR to a temporary runner directory in the canary workflow
• append that directory to GITHUB_PATH after running install.sh
• verify the installed binary is discoverable before checking openshell --version

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)