fix(pbs/pve): guard sed against missing /etc/apt/sources.list

PBS 4.x and fresh PVE 9 / Trixie installs no longer ship
/etc/apt/sources.list (deb822 only). Running 'sed -i ... /etc/apt/sources.list || true'
suppresses the exit code but still prints the noisy stderr message
'sed: can't read /etc/apt/sources.list: No such file or directory'
during the post-install routine.

Wrap the call with an existence check so the sed step is skipped cleanly
when the legacy file is absent.

Fixes #14174

tools/pve/pbs4-upgrade.sh

@@ -57,7 +57,9 @@ start_routines() {
   yes)
     msg_info "Switching to Debian 13 (Trixie) Sources"
     rm -f /etc/apt/sources.list.d/*.list
-    sed -i '/proxmox/d;/bookworm/d' /etc/apt/sources.list || true
+    if [ -f /etc/apt/sources.list ]; then
+      sed -i '/proxmox/d;/bookworm/d' /etc/apt/sources.list
+    fi
     cat >/etc/apt/sources.list.d/debian.sources <<EOF
 Types: deb
 URIs: http://deb.debian.org/debian

tools/pve/post-pbs-install.sh

@@ -188,7 +188,9 @@ start_routines_4() {
   yes)
     msg_info "Correcting Debian Sources (deb822)"
     rm -f /etc/apt/sources.list.d/*.list
-    sed -i '/proxmox/d;/bookworm/d' /etc/apt/sources.list || true
+    if [ -f /etc/apt/sources.list ]; then
+      sed -i '/proxmox/d;/bookworm/d' /etc/apt/sources.list
+    fi
     cat >/etc/apt/sources.list.d/debian.sources <<EOF
 Types: deb
 URIs: http://deb.debian.org/debian/

tools/pve/post-pve-install.sh

@@ -251,8 +251,10 @@ start_routines_9() {
       msg_info "Correcting Proxmox VE Sources (deb822)"
       # remove all existing .list files
       rm -f /etc/apt/sources.list.d/*.list
-      # remove bookworm and proxmox entries from sources.list
+      # remove bookworm and proxmox entries from sources.list (if it exists)
-      sed -i '/proxmox/d;/bookworm/d' /etc/apt/sources.list || true
+      if [ -f /etc/apt/sources.list ]; then
+        sed -i '/proxmox/d;/bookworm/d' /etc/apt/sources.list
+      fi
       # Create new deb822 sources
       cat >/etc/apt/sources.list.d/debian.sources <<EOF
 Types: deb