run_skylab_suite: allow an env var to specify SKYLAB_TOOL location

This improves the testability of run_suite_skylab.

CQ-DEPEND=CL:1531313
BUG=chromium:940661
TEST=./bin/run_suite_skylab in non-dryrun mode

Change-Id: I788ac4eee3be22f4ad3397dc90205c4156ff1d7b
Reviewed-on: https://chromium-review.googlesource.com/1531316
Commit-Ready: Aviv Keshet <akeshet@chromium.org>
Tested-by: Aviv Keshet <akeshet@chromium.org>
Reviewed-by: Xixuan Wu <xixuan@chromium.org>
diff --git a/venv/skylab_suite/suite_runner.py b/venv/skylab_suite/suite_runner.py
index ee88056..c5325e6 100644
--- a/venv/skylab_suite/suite_runner.py
+++ b/venv/skylab_suite/suite_runner.py
@@ -163,9 +163,10 @@
     @return the swarming task id of this task.
     """
     logging.info('Creating task for test %s', test_spec.test.name)
+    skylab_tool_path = os.environ.get('SKYLAB_TOOL', SKYLAB_TOOL)
 
     cmd = [
-        SKYLAB_TOOL, 'create-test',
+        skylab_tool_path, 'create-test',
         '-board', test_spec.board,
         '-image', test_spec.build,
         ]