Always add bootc install config

bootc isn't in C9S yet, but it is in the -dev images.  Add
the install config unconditionally so that `bootc install`
works in the `centos-bootc-dev` image.

tier-0/bootc-config.yaml

@@ -0,0 +1,10 @@
+# Configuration for bootc
+postprocess:
+  # XFS is our default filesystem
+  - |
+    #!/usr/bin/env bash
+    mkdir -p /usr/lib/bootc/install/
+    cat > /usr/lib/bootc/install/20-rhel.toml << EOF
+    [install]
+    root-fs-type = "xfs"
+    EOF