archiso hook: replace getarg with direct /proc/cmdline parsing

getarg's trailing-= convention varies by mkinitcpio version and was
returning empty even when archisosearchuuid was present in cmdline.
Parse /proc/cmdline directly in ash — no external tools, no version
sensitivity.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/profile/airootfs/usr/lib/initcpio/hooks/archiso

@@ -2,15 +2,19 @@
 
 archiso_mount_handler() {
     local _new_root="$1"
-    local _uuid _label _basedir _dev _img
+    local _uuid _label _basedir _dev _img _param
 
-    _uuid="$(getarg archisosearchuuid=)"
-    _label="$(getarg archisolabel=)"
-    _basedir="$(getarg archisobasedir=)"
+    for _param in $(cat /proc/cmdline); do
+        case "$_param" in
+            archisosearchuuid=*) _uuid="${_param#archisosearchuuid=}" ;;
+            archisolabel=*)      _label="${_param#archisolabel=}" ;;
+            archisobasedir=*)    _basedir="${_param#archisobasedir=}" ;;
+        esac
+    done
     : "${_basedir:=arch}"
 
     if [ -z "${_uuid}" ] && [ -z "${_label}" ]; then
-        echo "archiso: neither archisosearchuuid nor archisolabel set in kernel cmdline"
+        echo "archiso: neither archisosearchuuid nor archisolabel in kernel cmdline"
         return 1
     fi