archinstall_adapter: pass Path (not str) to Installer

archinstall 4.3's Installer.__init__ types target as Path and uses it
directly (self.target / 'boot', etc.). Passing str(mountpoint) makes
self.target a str, so every internal path operation raises
'unsupported operand type(s) for /: str and str' — most visibly during
set_mirrors, which constructs target / MIRRORLIST.relative_to_root().

Drop the str() cast; pass the Path through as-is.

configs/airootfs/usr/share/omarchy-iso/orchestrator/archinstall_adapter.py

@@ -99,7 +99,7 @@ def open_installer(
     if not arch_config.disk_config:
         raise RuntimeError("disk_config missing from arch config")
     with Installer(
-        str(mountpoint),
+        mountpoint,
         arch_config.disk_config,
         kernels=arch_config.kernels,
         silent=silent,