Move custom step shell snippets to .sh scripts

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_

Inspect the repository's GitHub Actions steps containing custom shell commands; no specific file or test is named in the issue. Identify which snippets should become .sh scripts, preserve the workflow behavior, and verify the resulting scripts and actions run successfully.

Written by the indexing model from the issue text.