auto_updater_transfer: Fix issues with ssh fallback

_GetCurlCmdForPayloadDownload() is mostly called for downloading files
into DUT. But there is one instance that it is used for downloading the
file (payload properties file) into the drone. However, this function
strips the 'payloads/' from the beginning of the payload file name and for
this specific instance of its use, it should not. This CL fixes that by
recreating proper variables to address this shortcoming.

BUG=None
TEST=run_pytest
TEST=test_that --args="target_release=13280.0.0 target_payload_uri='gs://chromeos-releases/dev-channel/reef/13290.0.0/payloads/chromeos_13290.0.0_reef_dev-channel_full_test.bin-gvswkobwg43gf4byzyw75rj4io3sl5y5' source_release=13280.0.0 source_payload_uri='gs://chromeos-releases/dev-channel/reef/13290.0.0/payloads/chromeos_13290.0.0_reef_dev-channel_full_test.bin-gvswkobwg43gf4byzyw75rj4io3sl5y5' update_type=full" chromeos6-row4-rack9-host3.cros.corp.google.com autoupdate_EndToEndTest

Change-Id: I6a6f267b1a2e31a4551790d13edb1f2f7be0f7e9
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2254678
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Sanika Kulkarni <sanikak@chromium.org>
Reviewed-by: Allen Li <ayatane@chromium.org>
Commit-Queue: Amin Hassani <ahassani@chromium.org>
diff --git a/lib/auto_updater_transfer.py b/lib/auto_updater_transfer.py
index 235278b..b1ce21c 100644
--- a/lib/auto_updater_transfer.py
+++ b/lib/auto_updater_transfer.py
@@ -553,6 +553,16 @@
         cmd = self._GetCurlCmdForPayloadDownload(
             payload_dir=self._tempdir, build_id=self._payload_dir,
             payload_filename=payload_props_filename)
+
+        # _GetCurlCmdForPayloadDownload removes the 'payloads/' prefix from the
+        # beginning of the payload_props_filename before creating the
+        # command. So here we need to remove that too.
+        prefix = 'payloads/'
+        if payload_props_filename.startswith(prefix):
+          payload_props_path = os.path.join(
+              self._tempdir,
+              payload_props_filename[len(prefix):])
+
         try:
           retry_util.RunCurl(cmd[1:])
         except retry_util.DownloadError as e: