composefs: Skip boot entry duplicate check during fresh installs

The find_vmlinuz_initrd_duplicates function looks at the host's
/sysroot/state/deploy directory to find existing deployments that
share the same kernel/initrd. However, during a fresh install to
a target disk (e.g., via --via-loopback), this causes a failure:

1. The host has existing deployments in /sysroot/state/deploy
2. find_vmlinuz_initrd_duplicates returns those entries
3. The code tries to find matching boot entries in the target's /boot
4. The target's /boot is empty → "Shared boot binaries not found"

Fix this by only checking for duplicates during upgrades (is_upgrade
== true). For fresh installs, the target boot partition is empty
anyway, and the duplicate check looks at the wrong state directory
(host vs target).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
Signed-off-by: Xiaofeng Wang <henrywangxf@me.com>

Author: henrywang

crates/lib/src/bootc_composefs/boot.rs

@@ -694,7 +694,17 @@ pub(crate) fn setup_composefs_bls_boot(
                     options: Some(cmdline_refs),
                 });
 
-            match find_vmlinuz_initrd_duplicates(&boot_digest)? {
+            // Only check for duplicate boot entries during upgrades.
+            // For fresh installs, the target boot partition is empty, so there's
+            // nothing to share. Additionally, find_vmlinuz_initrd_duplicates looks
+            // at the host's /sysroot/state/deploy, not the target's state.
+            let duplicates = if is_upgrade {
+                find_vmlinuz_initrd_duplicates(&boot_digest)?
+            } else {
+                None
+            };
+
+            match duplicates {
                 Some(shared_entries) => {
                     // Multiple deployments could be using the same kernel + initrd, but there
                     // would be only one available