Jenkins: T3664: specify PYTHONDONTWRITEBYTECODE environment variable

The new buildsystem also creates python bytecode which has the drawback that
the Jenkins call to "sudo make purge" and "deleteDir()" can not remove the
files under vyos-build/scripts/__pycache__/ and gives:

vyos_build_defaults.cpython-39.pyc: Operation not permitted
vyos_build_utils.cpython-39.pyc: Operation not permitted

This commit disables python byte-code generation on ISO builds as there is no
performance impact as the script is called only once anyway.

Jenkinsfile

@@ -81,6 +81,9 @@ pipeline {
                     triggeredBy cause: "UserIdCause"
                 }
             }
+            environment {
+                PYTHONDONTWRITEBYTECODE = 1
+            }
             steps {
                 script {
                     // Display Git commit Id used with the Jenkinsfile on the Job "Build History" pane