ciscript: don't use an empty string as a number

An empty string is not a valid number, and some shells complain.

Check instead if $COVERITY is non-empty, which is a common convention
and what we're doing anyway.

script/cibuild.sh

 #!/bin/sh
 
-if [ "$COVERITY" -eq 1 ];
+if [ -n "$COVERITY" ];
 then
 	./script/coverity.sh;
 	exit $?;