Set --logfile for all kyua invocations

When kyua is called without the --logfile command line option, the log
file is created at a default location which is derived from the HOME
environment variable.  On FreeBSD GitLab CI runners, /home is a
read-only directory and thus kyua invocations not using the --logfile
option fail when HOME is set to something beneath /home.  Set --logfile
to /dev/null for all kyua invocations whose logs are irrelevant in order
to prevent kyua failures caused by HOME being non-writable.

(cherry picked from commit 1bffa602ba)

.gitlab-ci.yml

@@ -183,7 +183,7 @@ stages:
     when: on_failure
 
 .kyua_report: &kyua_report_html |
-  kyua report-html \
+  kyua --logfile /dev/null report-html \
        --force \
        --results-file "$KYUA_RESULT" \
        --results-filter "" \

unit/unittest.sh.in

@@ -13,7 +13,7 @@ then
 	${KYUA} -v parallelism="${TEST_PARALLEL_JOBS:-1}" --logfile kyua.log --loglevel debug test --results-file "${KYUA_RESULT:-NEW}"
 	status=$?
 
-	${KYUA} report --results-file "${KYUA_RESULT:-LATEST}"
+	${KYUA} --logfile /dev/null report --results-file "${KYUA_RESULT:-LATEST}"
 
 	if [ "${status}" -eq "0" ]
 	then