* Shellcheck on build_linux.sh and tiny refactor
`shellcheck -e SC1090 build_linux.sh` and fixed the issues.
Also fixed the usage command to return an error, simplify directory
creation to use `mkdir -p`, and made printing of the cmake commands
consistent instead of having three different ways of doing it.
* Reorder functions in more pleasing way
* Add shellcheck to workflows
This is the beginning of adding some safety nets for making
changes. Currently it just runs shellcheck against `build_linux.sh`.
You can see it working at
https://github.com/coryrc/OrcaSlicer/actions/workflows/shellcheck.yml
I forked and tested it running in PRs and against pushes to HEAD.
* Rerun shellcheck
Missed quotes on the merge
