[lldb/test] Fix TestLoadUnload

Summary:
[lldb/test] Fix TestLoadUnload failure introduced in e0dbd025

It seems that `env_cmd_string` is declared and used few lines under this
self.runCmd expression. I guess this is some left-over from an older
version.

Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>

Reviewers: labath

Reviewed By: labath

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D78094