Move custom step shell snippets to .sh scripts

[frostebite]

          When I see commands in a github action step, I'm always wondering if it's better to have them in the yaml or expose a shell script so we gain syntax highlighting and we can run shellcheck or other tools like that against them. _just thinking out loud here_

Originally posted by @GabLeRoux in https://github.com/game-ci/unity-builder/pull/479#discussion_r1081635508