COMP: Add shell: bash to Download ITK step for Windows compatibility

The Download ITK step uses $ITK_BUILD_ROOT (bash syntax) but has no
explicit shell: directive. On Windows, GitHub Actions defaults to pwsh
where $ITK_BUILD_ROOT is undefined (needs $env:ITK_BUILD_ROOT). The cd
becomes a no-op, ITK gets cloned into the repo checkout directory
instead of the build root, and cmake fails with "source directory does
not exist".

Adding shell: bash ensures the step uses Git Bash on Windows, where
$ITK_BUILD_ROOT correctly reads the environment variable.

Author: hjmjohnson

.github/workflows/build-test-cxx.yml

@@ -141,6 +141,7 @@ jobs:
         sudo xcode-select -s "/Applications/Xcode_16.2.0.app"
 
     - name: Download ITK
+      shell: bash
       run: |
         cd "$ITK_BUILD_ROOT"
         git clone https://github.com/InsightSoftwareConsortium/ITK.git