test/functional: Fix Aspeed buildroot tests

test/functional: Fix Aspeed buildroot tests

Since commit 786bc2255256, cached asset files are read-only. Adjust
the QEMU command line for buildroot tests to reflect the new
constraint on the flash drive.

Fixes: f04cb2d00d5c ("tests/functional: Convert most Aspeed machine tests")
Suggested-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20241105132339.2967202-1-clg@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>