ci: Update bcvk to v0.6.0 for --karg support

The test-integration-cfs job requires bcvk --karg support which was
added in v0.6.0. Update from v0.5.3 to v0.6.0.

Assisted-by: Claude Code (Sonnet 4.5)

Author: cgwalters

.github/actions/bootc-ubuntu-setup/action.yml

@@ -77,7 +77,7 @@ runs:
       shell: bash
       run: |
         set -xeuo pipefail
-        export BCVK_VERSION=0.5.3
+        export BCVK_VERSION=0.6.0
         /bin/time -f '%E %C' sudo apt install -y libkrb5-dev pkg-config libvirt-dev genisoimage qemu-utils qemu-kvm virtiofsd libvirt-daemon-system
         # Something in the stack is overriding this, but we want session right now for bcvk
         echo LIBVIRT_DEFAULT_URI=qemu:///session >> $GITHUB_ENV