Run shellcheck on build_linux.sh and add it to workflows (#10317)

* 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

.github/workflows/shellcheck.yml

@@ -0,0 +1,38 @@
+on:
+  push:
+    paths:
+      - build_linux.sh
+  pull_request:
+    paths:
+      - build_linux.sh
+  workflow_dispatch: # allows for manual dispatch
+
+name: "Shellcheck"
+permissions: {}
+
+jobs:
+  shellcheck:
+    name: Shellcheck
+    runs-on: ubuntu-latest
+    steps:
+      - name: Cache shellcheck download
+        id: cache-shellcheck-v0_11
+        uses: actions/cache@v4
+        with:
+          path: ~/shellcheck
+          key: ${{ runner.os }}-shellcheck-v0_11
+
+      - name: Download shellcheck
+        if: steps.cache-shellcheck-v0_11.outputs.cache-hit != 'true'
+        shell: bash
+        env:
+          INPUT_VERSION: "v0.11.0"
+        run: |
+          curl -L#o ~/sc.tar.xz "https://github.com/koalaman/shellcheck/releases/download/${INPUT_VERSION}/shellcheck-${INPUT_VERSION}.linux.x86_64.tar.xz"
+          tar -xvf ~/sc.tar.xz -C ~
+          mv ~/shellcheck-"${INPUT_VERSION}"/shellcheck ~/shellcheck
+
+      - uses: actions/checkout@v4
+
+      - name: Shellcheck build_linux.sh
+        run: ~/shellcheck -e SC1090 build_linux.sh