ci: twister_tests_blackbox: Switch to CI image v0.26.9

This commit updates the twister_tests_blackbox workflow to use the CI image
v0.26.9, in order to pull in the Zephyr SDK 0.16.5-1 release and keep the
image and SDK versions in sync with the rest of the CI workflows.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>
1 file changed