composefs: Skip boot binary dedup check during fresh install

find_vmlinuz_initrd_duplicates() reads the host's /sysroot/state/deploy
to find existing deployments with matching boot digests. During a fresh
install (not an upgrade), the host's own deployment data matches the
boot digest, but the target disk has no existing boot entries yet. This
caused "Shared boot binaries not found" when installing outside a
container on a composefs+grub system.

Fix by only performing the duplicate check during upgrades, where
shared boot binaries on the target actually exist.

Assisted-by: Claude Code (Opus 4)
Signed-off-by: ckyrouac <ckyrouac@redhat.com>

Author: ckyrouac

crates/lib/src/bootc_composefs/boot.rs

@@ -705,7 +705,14 @@ pub(crate) fn setup_composefs_bls_boot(
                     options: Some(cmdline_refs),
                 });
 
-            match find_vmlinuz_initrd_duplicates(&boot_digest)? {
+            // Only check for shared boot binaries during upgrades. During a
+            // fresh install the target has no existing entries, and the host's
+            // /sysroot/state/deploy would incorrectly match.
+            match if is_upgrade {
+                find_vmlinuz_initrd_duplicates(&boot_digest)?
+            } else {
+                None
+            } {
                 Some(shared_entries) => {
                     // Multiple deployments could be using the same kernel + initrd, but there
                     // would be only one available