CI: Replace cat line when writing inventory for container image build

I don't know where this was removed, possibly in a bad merge.

Author: markgoddard

.github/workflows/stackhpc-container-image-build.yml

@@ -135,6 +135,7 @@ jobs:
 
       - name: Configure localhost as a seed
         run: |
+          cat > src/kayobe-config/etc/kayobe/environments/ci-builder/inventory/hosts << EOF
           # A 'seed' host used for building images.
           # Use localhost for container image builds.
           [seed]