Use `terminus-github-actions` and Sage install script cleanup

[jazzsequence]

• replaces (deprecated, duplicate) pantheon-systems/action-install-terminus with pantheon-systems/terminus-github-actions
• updates the output of the composer-diff comment
• generates a random theme name for the sage install (rather than just sage-test)
• adds a wait step before switching to sftp mode for the add_symlink step
• checks if the /files/cache directory exists before creating it
• updates multisite check logic to use wp config is-true (rather than running theme enable regardless)
• bails early if running in CI (last several steps are not needed for CI testing)