mkarchiso: run _cms_sign_artifact only once

Since `_build_buildmode_netboot` runs `_build_iso_base` which runs
`_prepare_airootfs_image`, the call to `_cms_sign_artifact` in
`_build_buildmode_netboot` can be removed.

Fixes #197

CHANGELOG.rst

@@ -17,6 +17,8 @@ Deprecated
 Fixed
 -----
 
+- Sign the root file system image only once.
+
 Removed
 -------
 

archiso/mkarchiso

@@ -1813,7 +1813,6 @@ _build_buildmode_netboot() {
 
     if [[ -v cert_list ]]; then
         _run_once _sign_netboot_artifacts
-        _cms_sign_artifact "${airootfs_image_filename}"
     fi
     _run_once _export_netboot_artifacts
 }