ci: save and show both stdout and stderr

By using the `|&` operator we send both to `tee`.

Signed-off-by: Jakub Sokołowski <jakub@status.im>

ci/Jenkinsfile.tests

@@ -54,10 +54,10 @@ pipeline {
     stage('Checks') {
       parallel {
         stage('Lint') {
-          steps { sh "make lint > ${LOG_FILE}" }
+          steps { sh "make lint 2>&1 | tee ${LOG_FILE}" }
         }
         stage('Tests') {
-          steps { sh "make test >> ${LOG_FILE}" }
+          steps { sh "make test 2>&1 | tee -a ${LOG_FILE}" }
         }
       }
     }