Fixed broken iso build from last commit

Makefile

@@ -221,6 +221,7 @@ check_login_var:
 	$(call check_defined, LOGIN)
 
 check_nexus_exam_vars:
+	$(call check_defined, CERT)
 	$(call check_defined, EXAM_USER)
 	$(call check_defined, EXAM_PWD)
 

live_exam_os/nexus_exam_builder/Dockerfile

@@ -8,5 +8,5 @@ RUN apt-get update && apt-get install -y golang-go make ca-certificates libglfw3
 RUN echo $server > /server
 RUN echo $exam_user > /exam_user
 RUN echo $exam_pwd > /exam_pwd
-RUN make build_nexus-exam SERVER=$server CERT=/nexus EXAM_USER=$exam_user EXAM_PWD=$exam_pwd
+RUN make build_nexus-exam SERVER=$server CERT=/nexus/ca-cert.pem EXAM_USER=$exam_user EXAM_PWD=$exam_pwd
 RUN ["sh"]