make signing depend on slowchecks being done

otherwise signing (at least on windows) fails because files are in use

Change-Id: Ida6a7d43dc74eb278fd79410b9c0a60f823c5933
Reviewed-on: https://gerrit.libreoffice.org/9176
Tested-by: David Tardon <dtardon@redhat.com>
Reviewed-by: David Tardon <dtardon@redhat.com>