tests: Use locally-built image in test-install-outside-container

The test was using `get_target_image` which returns the upstream
`docker://quay.io/centos-bootc/centos-bootc:stream10`. On composefs+grub
variants provisioned with an updated bootupd from copr, the upstream
image has stock bootupd with incompatible EFI update metadata, causing
the install to fail with "Failed to find EFI update metadata".

Switch to using `containers-storage:localhost/bootc` (the locally-built
image), matching the pattern used by test-32, test-37, and test-38.
The locally-built image has the updated bootupd with compatible metadata.

Assisted-by: Claude Code (Opus 4)
Signed-off-by: ckyrouac <ckyrouac@redhat.com>
Signed-off-by: Colin Walters <walters@verbum.org>

Author: ckyrouac

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

@@ -6,9 +6,10 @@
 use std assert
 use tap.nu
 
-# Use an OS-matched target image to avoid version mismatches
-# (e.g., XFS features created by newer mkfs.xfs not recognized by older grub2)
-let target_image = (tap get_target_image)
+# Use the locally-built image which has updated bootupd with compatible
+# EFI update metadata, matching the pattern used by test-32/37/38.
+bootc image copy-to-storage
+let target_image = "containers-storage:localhost/bootc"
 
 # setup filesystem
 mkdir /var/mnt