Fix for workspace cleanup in test-salt-formulas

It's need to use sudo because jenkind does not have permissions to remove
files/directories created by root in docker container.

Change-Id: I8a6449689f54c66ff2687c71543489f54a846e13
diff --git a/test-salt-formulas-pipeline.groovy b/test-salt-formulas-pipeline.groovy
index b21140c..a850a3e 100644
--- a/test-salt-formulas-pipeline.groovy
+++ b/test-salt-formulas-pipeline.groovy
@@ -76,9 +76,7 @@
       stage("checkout") {
         if (fileExists('tests/build')) {
           echo 'Cleaning from previous build...'
-          dir('tests/build') {
-            deleteDir()
-          }
+          sh ('sudo rm -rf tests/build')
         }
 
         if (gerritRef) {