add tag to spack in github workflow

Author: DanecLacey

.github/workflows/single-platform.yml

@@ -14,7 +14,7 @@ jobs:
     - uses: actions/checkout@v4
 
     - name: Setup Spack
-      uses: spack/actions/setup-spack
+      uses: spack/actions/setup-spack@main 
       with:
         spack-version: latest