update.sh: Fixes bug#13138 - root/host certificate set fails to be created

update.sh: Fixes bug#13138 - root/host certificate set fails to be created

- The fix applied in vpnmain.cgi only adds the unique_subject = yes to the index.txt.attr
   file after the first time that the root/host certificates are attempted to be created.
- Without this line in update.sh, the first attempt to create the root/host certificate set
   will still have the original error code. If the creation is attempted again then it will
   work because the unique_subject = yes will have then been added into the file.
- This patch ensures that the first attempt to create a root/host certificate set in CU175
   will work.
- Confirmed on vm testbed with freshly updated CU175.

Fixes: Bug#13138
Tested-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>