refactor(jenkinsfile): fix syntax

Signed-off-by: zontreck <tarapiccari@gmail.com>

Jenkinsfile

@@ -16,12 +16,14 @@ pipeline {
             }
 
             steps {
-                script '''
-                #!/bin/bash
+                script {
+                    sh '''
+                    #!/bin/bash
 
-                git clean -xfd
-                git reset --hard
-                '''
+                    git clean -xfd
+                    git reset --hard
+                    '''
+                }
             }
         }
 
@@ -31,11 +33,13 @@ pipeline {
             }
 
             steps {
-                script '''
-                #!/bin/bash
+                script {
+                    sh '''
+                    #!/bin/bash
 
-                mkarchiso -v -w work -o out server
-                '''
+                    mkarchiso -v -w work -o out server
+                    '''
+                }
             }
         }
     }