ci: silence SC2187 on the busybox-sh shebangs (shellcheck 0.9 vs 0.10)

ubuntu-latest ships shellcheck 0.9.0, which warns that ash scripts are
checked as dash; 0.10 (local) does not. shell=dash is the accurate dialect
for busybox ash. Verified clean under BOTH versions; boot smoke re-run.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018HUayid7W5w7jBdb9Rrj1K

qemu/rootfs/init

@@ -1,4 +1,5 @@
 #!/bin/busybox sh
+# shellcheck shell=dash
 # WardenOS QEMU device sim: initramfs /init (PID 1).
 #
 # Phase-1 duty: prove the kernel booted on -M virt — print the sentinel the