ci: Use c9s for install-outside-container tests

The newer bootupd hasn't been rolled out to this image yet,
so this should avoid version skew problems.

See coreos/bootupd#995

Signed-off-by: Colin Walters <walters@verbum.org>

Author: cgwalters

tmt/tests/booted/test-install-outside-container.nu

@@ -10,7 +10,7 @@ use tap.nu
 # this test in theory independent of starting from a bootc host,
 # but also because it's useful to test "skew" between the bootc binary
 # doing the install and the target image.
-let target_image = "docker://quay.io/centos-bootc/centos-bootc:stream10"
+let target_image = "docker://quay.io/centos-bootc/centos-bootc:stream9"
 
 # setup filesystem
 mkdir /var/mnt

tmt/tests/booted/test-install-unified-flag.nu

@@ -13,7 +13,7 @@ use tap.nu
 
 # Use a generic target image to test skew between the bootc binary doing
 # the install and the target image
-let target_image = "docker://quay.io/centos-bootc/centos-bootc:stream10"
+let target_image = "docker://quay.io/centos-bootc/centos-bootc:stream9"
 
 def main [] {
     tap begin "install with experimental unified storage flag"