mkarchiso: Do not run _build_iso_base once

archiso/mkarchiso:
Do not run `_build_iso_base` with `_run_once`, as it sets variables which are important to functions that are called
after it.

archiso/mkarchiso

@@ -1241,7 +1241,7 @@ _build_buildmode_bootstrap() {
 _build_buildmode_netboot() {
     local run_once_mode="${buildmode}"
 
-    _run_once _build_iso_base
+    _build_iso_base
     if [[ -v cert_list ]]; then
         _run_once _sign_netboot_artifacts
     fi
@@ -1252,7 +1252,7 @@ _build_buildmode_netboot() {
 _build_buildmode_iso() {
     local image_name="${iso_name}-${iso_version}-${arch}.iso"
     local run_once_mode="${buildmode}"
-    _run_once _build_iso_base
+    _build_iso_base
     _run_once _build_iso_image
 }