github: Don't fail LGTM test with an error if variables are missing

That's because forks are currently not allowed to be analyzed by LGTM (unlike
with SonarCloud) so this check can't actually be successful for forks even if
variables are defined.

References strongswan/strongswan#328.

scripts/test.sh

@@ -325,7 +325,7 @@ lgtm)
 	if [ -z "$LGTM_PROJECT" -o -z "$LGTM_TOKEN" ]; then
 		echo "The LGTM_PROJECT and LGTM_TOKEN environment variables" \
 			 "are required to run this test"
-		exit 1
+		exit 0
 	fi
 	DEPS="jq"
 	if test -z "$1"; then